> ## Content Index > Fetch the complete content index at: https://ijcci.net/llms.txt > Use this file to discover other available public pages before exploring further. # Best Deck Stain to Prevent Mildew: Wet & Forget Leads - URL: https://ijcci.net/best-deck-stain-prevent-mildew/ - Published: 2026-09-07T06:54:13.000Z - Updated: 2026-09-07T09:49:20.000Z - Description: A stain film only lasts as long as the wood under it stays clean. Six mildew stain removers compared by coverage, application method and what each one asks of you before you touch a can of finish. - Author: Dane Pruitt - Tags: Cleaning & Laundry, #Deck Care, #Outdoor Cleaning The best deck stain to prevent mildew is only as good as the wash underneath it, and the pick I'd start with is Wet & Forget Outdoor Cleaner & Stain Remover, whose 64 oz bottle treats 60 to 180 square feet and needs no rinsing at all. If you want the black stains visibly gone before you open a can of finish, 30 SECONDS Pro Mold and Mildew Stain Remover comes in a 128 fl oz jug instead. Money in this category buys coverage and reach, not magic: the cheapest picks attach to a hose, the middle ones bring stronger formulas, and the top of the range adds concentrate plus a brush. In this guide 1. [Best deck stain to prevent mildew at a glance](#best-deck-stain-to-prevent-mildew-at-a-glance) 2. [How I screened these picks and what the search actually returns](#how-i-screened-these-picks-and-what-the-search-actually-returns) 3. [Why the best deck stain to prevent mildew starts with the wash](#why-the-best-deck-stain-to-prevent-mildew-starts-with-the-wash) 4. [1\. Wet & Forget Outdoor Cleaner & Stain Remover — Best overall](#1-wet-forget-outdoor-cleaner-stain-remover-%E2%80%94-best-overall) 5. [2\. 30 SECONDS Pro Mold and Mildew Stain Remover — Most versatile](#2-30-seconds-pro-mold-and-mildew-stain-remover-%E2%80%94-most-versatile) 6. [3\. Moldex Instant Outdoor Cleaner with Bleach and Hose End — Best budget pick](#3-moldex-instant-outdoor-cleaner-with-bleach-and-hose-end-%E2%80%94-best-budget-pick) 7. [4\. RMR Deck, Patio, & Fence Wash — Best upgrade](#4-rmr-deck-patio-fence-wash-%E2%80%94-best-upgrade) 8. [5\. 30 SECONDS Mold and Mildew Stain Remover — Best minimal setup](#5-30-seconds-mold-and-mildew-stain-remover-%E2%80%94-best-minimal-setup) 9. [6\. DeckMax Premium Deck Cleaner Kit — Best premium](#6-deckmax-premium-deck-cleaner-kit-%E2%80%94-best-premium) 10. [Which application method suits your deck?](#which-application-method-suits-your-deck) 11. [How much surface does one bottle actually cover?](#how-much-surface-does-one-bottle-actually-cover) 12. [What goes wrong when you stain over mildew?](#what-goes-wrong-when-you-stain-over-mildew) 13. [How often do you need to repeat this?](#how-often-do-you-need-to-repeat-this) 14. [The verdict](#the-verdict) 15. [FAQ](#faq) ## Best deck stain to prevent mildew at a glance | Pick | Size | Application | Price | | ------------------------------------------------------------------------------------------------------------------- | ---------- | --------------------------- | ---------------------------------------------------------------- | | **Best overall**[Wet & Forget Outdoor Cleaner & Stain Remover](https://www.amazon.com/dp/B01MA6BG4U?tag=ijcci-20) | 64 fl oz | Trigger sprayer, no rinse | [Check Price](https://www.amazon.com/dp/B01MA6BG4U?tag=ijcci-20) | | **Most versatile**[30 SECONDS Pro Mold and Mildew Stain Remover](https://www.amazon.com/dp/B0D217V9KT?tag=ijcci-20) | 128 fl oz | Ready-to-use refill | [Check Price](https://www.amazon.com/dp/B0D217V9KT?tag=ijcci-20) | | **Best budget pick**[Moldex Instant Outdoor Cleaner with Bleach](https://www.amazon.com/dp/B0GPRHM4KT?tag=ijcci-20) | 64 fl oz | Hose-end, 40 ft reach claim | [Check Price](https://www.amazon.com/dp/B0GPRHM4KT?tag=ijcci-20) | | **Best upgrade**[RMR Deck, Patio, & Fence Wash](https://www.amazon.com/dp/B0CW25QR49?tag=ijcci-20) | 64 fl oz | Hose-end adapter, rinse off | [Check Price](https://www.amazon.com/dp/B0CW25QR49?tag=ijcci-20) | | **Best minimal setup**[30 SECONDS Mold and Mildew Stain Remover](https://www.amazon.com/dp/B0F1W5W7RZ?tag=ijcci-20) | 1.3 gallon | Battery spray wand, 1:1 mix | [Check Price](https://www.amazon.com/dp/B0F1W5W7RZ?tag=ijcci-20) | | **Best premium**[DeckMax Premium Deck Cleaner Kit](https://www.amazon.com/dp/B085P25DX7?tag=ijcci-20) | 64 fl oz | Concentrate plus brush | [Check Price](https://www.amazon.com/dp/B085P25DX7?tag=ijcci-20) | ## How I screened these picks and what the search actually returns Here's the thing you need to know before reading another word. Search for the best deck stain to prevent mildew and Amazon returns cleaners, not stains. Every one of the six products below is a mold, mildew and algae stain remover. None of them is a pigmented finish, and none of them can be claimed to seal or colour your boards. I weighted three criteria, in this order. 1. **Coverage per bottle**, because a deck is a large flat surface and running short mid-job leaves a visible line. 2. **Application method**, because a hose-end sprayer, a trigger bottle and a battery wand suit three very different decks. 3. **Surface compatibility as stated by the maker**, since composite decking and natural wood do not always accept the same formula. Those three matter more than anything else here because the published data is thin. Two picks list a warranty or a released year; four do not. One lists dimensions to a tenth of an inch; another lists nothing but a weight and a model number. So I ranked on what every listing does state and on what the manufacturers claim about surfaces, attributed as claims. What got cut: anything that read as an interior-only bathroom product, and anything whose listing named no outdoor surface. That leaves six picks spanning 64 fl oz single bottles up to a 1.3 gallon kit. ## Why the best deck stain to prevent mildew starts with the wash Most guides to the best deck stain to prevent mildew compare mildewcide additives and film thickness. That isn't what decides the outcome. What decides it is whether the boards were biologically clean when the finish went on. Mildew and algae feed on the organic film that settles on horizontal wood: pollen, leaf tannin, dust, the grey layer of degraded lignin. Seal that film under a stain and you've built the colony a roof. The black comes back through the finish in one season, and now you can't reach it without stripping. So the useful question isn't which finish resists mildew best. It's how thoroughly you can remove the existing growth and its food source first. That's what these six products do, and it's why coverage figures matter more than any claim on the label. Two of the picks make a point of not needing rinsing or scrubbing. Wet & Forget states you apply generously and do not rinse, letting wind and rain finish the job. Moldex claims no brushing or pressure washing. Three others expect a rinse or a scrub. That split is the real fork in the road. ## 1\. Wet & Forget Outdoor Cleaner & Stain Remover — Best overall **A bleach-free, non-acidic formula you spray on and walk away from, which is exactly what a shaded deck needs on repeat.** [![64 oz Wet and Forget trigger-spray bottle of outdoor mold and mildew stain remover — the no-rinse formula that treats 60 to 180 square feet](https://m.media-amazon.com/images/I/41JGAOwPLzL._SL500_.jpg)](https://www.amazon.com/dp/B01MA6BG4U?tag=ijcci-20) **Best for:** shaded decks where mildew returns every season **Skip it if:** you need the stains gone this weekend This is the one buyers choose most often among the picks here that come in a 64 fl oz trigger bottle, and the reason is the application model. There's no rinsing. Wet & Forget says you apply generously to a dry surface on a cool day, skip the hose entirely, and let weather do the rest. The bottle weighs 4 lb and measures 9 x 3 x 6 in, so it stores on a garage shelf between uses. It's the only pick here that publishes both a released year of 2022 and a warranty. The timeline is the honest part. Wet & Forget states green stains clear in one to two weeks, black or heavy stains can take several months, and long-established staining may run 6 to 12 months. That's slow. It's also why it suits a maintenance rhythm rather than a rescue mission. Coverage is stated at 60 to 180 square feet per bottle, a wide band because porous and heavily stained wood drinks more. The trade-off against the hose-end picks is speed and reach. You're pulling a trigger by hand across every board, and you're waiting out the calendar. Against the concentrates, you get less total product per bottle. **Pros:** - Bleach-free and non-acidic per the maker - No rinsing step at all - Coverage stated at 60 to 180 sq ft - Compact 9 x 3 x 6 in bottle stores easily - Published warranty and 2022 release **Cons:** - Black stains may take several months to clear - Only one seller lists it - Hand-trigger application is slow across a large deck - Needs a 4 to 5 hour dry window before rain [Check Price on Amazon](https://www.amazon.com/dp/B01MA6BG4U?tag=ijcci-20) ## 2\. 30 SECONDS Pro Mold and Mildew Stain Remover — Most versatile **A 128 fl oz ready-to-use jug the maker rates at twice the strength of its standard outdoor cleaner, and it's the pick with the widest stated surface list.** [![128 fl oz jug of 30 SECONDS Pro mold and mildew stain remover — double the volume of every 64 oz bottle in this comparison](https://m.media-amazon.com/images/I/51jzWeZ-ShL._SL500_.jpg)](https://www.amazon.com/dp/B0D217V9KT?tag=ijcci-20) **Best for:** one bottle covering deck, siding and bathroom grout **Skip it if:** you only have a small landing to treat Volume is the headline. At 128 fl oz this is double every 64 fl oz bottle in the set, and it ships as a ready-to-use refill rather than a concentrate, so what you pour is what you spray. The jug weighs 8 lb and measures 10.6 x 4.92 x 7.95 in. It sells faster than any other pick here. 30 SECONDS lists an unusually broad surface range: drywall, wood, fences, decks, vinyl siding, concrete floors, kitchen and bathroom tile and grout, bathtub soap scum, showers and brick walls. That's a claim, not a test result, but it's the only listing here that spans indoor and outdoor use in one product. The company also claims the formula is 2X stronger than its standard outdoor cleaner and that no scrubbing is required, with brightening visible as you watch. Two things to weigh. There's no hose-end adapter and no sprayer listed, so you supply the applicator, and a refill jug is awkward to pour into a pump sprayer without a funnel. And unlike Wet & Forget, this one carries no published warranty or release year. You're buying volume and breadth, not documentation. **Pros:** - 128 fl oz, double the volume of the 64 oz picks - Ready to use, no mixing or dilution - Widest stated surface list in this set - Maker claims 2X strength versus its standard cleaner **Cons:** - No sprayer or applicator included - 8 lb jug is awkward to pour - No published warranty or release year - Overkill for a small balcony deck [Check Price on Amazon](https://www.amazon.com/dp/B0D217V9KT?tag=ijcci-20) ## 3\. Moldex Instant Outdoor Cleaner with Bleach and Hose End — Best budget pick **The cheapest pick here, and the only one publishing a spray reach figure: 40 feet from a garden hose.** [![Moldex 64 oz outdoor cleaner bottle with built-in hose-end sprayer — the pick claiming 40 feet of spray reach for siding and fences](https://m.media-amazon.com/images/I/41XlDA6tlzL._SL500_.jpg)](https://www.amazon.com/dp/B0GPRHM4KT?tag=ijcci-20) **Best for:** large decks and siding reached from a garden hose **Skip it if:** you have delicate plantings against the deck edge Reach is what this one sells. Moldex states a built-in hose-end sprayer delivering broad, even coverage up to 40 feet, which is the only distance figure any listing in this set publishes. On a deck with attached siding above it, that means you treat both from one standing position instead of dragging a ladder around. It's the cheapest of the six. The listing publishes only three specs: 64 fluid oz, count of 1, and model 369729\. No weight, no dimensions. Moldex claims the formula breaks down and lifts surface stains from mold, mildew, algae and dirt on decks, fences, brick, masonry, siding and stucco, with no brushing, scrubbing or pressure washing, and says it's safe around lawns and landscaping when used as directed. Note the bleach. It's named in the product title, and that's the trade-off against Wet & Forget's bleach-free chemistry. Bleach-based cleaners act faster on black staining and are harder on adjacent plants and on some fasteners. If your deck edges into a flower bed, that matters. If it's surrounded by lawn you're going to water anyway, less so. **Pros:** - Cheapest of the six picks - Built-in hose-end sprayer, nothing to buy - 40 ft stated spray reach, the only figure in this set - Fast-acting per the maker, no scrubbing claimed **Cons:** - Contains bleach, unlike the top pick - Only three specifications published - No weight or dimensions listed - Hose-end dilution gives you less control over strength [Check Price on Amazon](https://www.amazon.com/dp/B0GPRHM4KT?tag=ijcci-20) ## 4\. RMR Deck, Patio, & Fence Wash — Best upgrade **A commercial-grade wash with a hose-end adapter and the longest stated surface list for composite decking specifically.** [![RMR Deck Patio and Fence Wash 64 oz bottle with hose-end adapter — the commercial-grade pick that names composite decking and PVC among its surfaces](https://m.media-amazon.com/images/I/41bdUvDf+OL._SL500_.jpg)](https://www.amazon.com/dp/B0CW25QR49?tag=ijcci-20) **Best for:** composite decking and fence panels in one pass **Skip it if:** you want a formula you can leave to weather in RMR sits at the second-cheapest position here while carrying a hose-end adapter, which is the combination that makes it the upgrade over Moldex rather than a sidestep. The bottle is 64 fl oz and weighs 4 lb, model RMRDW64oz. What separates it inside this set is the composite naming. RMR lists composite decking, fencing, natural deck, patio furniture, vinyl, brick, stucco, aluminum, concrete, trim, PVC, masonry, playground equipment and painted surfaces. If your deck is composite with aluminum railings and PVC trim, that's three materials named on one label. The company says its industrial-strength products are used by hospitals, municipalities, schools, hotels and marine operators, and describes a prep, clean and rinse cycle finishing in minutes. That rinse cycle is the catch. RMR is not a leave-it product. You spray, you wait, you flush the surface with water, and on a raised deck that means managing runoff and thinking about what's planted below. RMR also recommends eye protection and rubber gloves. The listing publishes no dimensions and no warranty, and it's the slowest seller of the six here, which tells you nothing about the formula but does mean fewer shoppers have chosen it. **Pros:** - Names composite, PVC and aluminum specifically - Hose-end adapter built in - Second-cheapest of the six - Fast prep, clean and rinse cycle per the maker **Cons:** - Requires a rinse step, unlike the top pick - Eye protection and gloves recommended - No dimensions or warranty published - Slowest seller of the six here [Check Price on Amazon](https://www.amazon.com/dp/B0CW25QR49?tag=ijcci-20) ## 5\. 30 SECONDS Mold and Mildew Stain Remover — Best minimal setup **A 1.3 gallon concentrate with a battery-powered spray wand, so you're not pumping a canister across a hundred boards.** [![30 SECONDS 1.3 gallon mold and mildew remover with battery-powered spray wand — the largest volume in this set at 11.5 lb shipped](https://m.media-amazon.com/images/I/418YMezTzQL._SL500_.jpg)](https://www.amazon.com/dp/B0F1W5W7RZ?tag=ijcci-20) **Best for:** reaching second-storey siding and long fence runs **Skip it if:** you want to skip mixing entirely The wand is the whole argument. This kit ships at 11.5 lb, the heaviest here by nearly 4 lb, because it's a 1.3 gallon container plus a battery-powered spray wand. No trigger fatigue, no hose to drag, no pump to prime. On a long deck-and-fence job that's the difference between an afternoon and a weekend. It's a concentrate mixed 1:1 with water, which roughly doubles your working volume from the 1.3 gallon starting point. 30 SECONDS lists decks, composite and stained surfaces, concrete, bricks and masonry, gutters, siding, canvas, plastic and stucco. The company says the formula is safe around landscaping, plants and lawn when used as directed, and that it breaks down once rinsed into soil. That plant claim matters if you're spraying near beds. The published data is the thinnest in the set: a weight and a model number, model 30S149ZBP, and nothing else. No dimensions, no colour, no warranty, no released year. And it sits at the more expensive end of the six for a product that still asks you to mix. Against Wet & Forget, you trade a walk-away formula for speed and reach. Against the Pro jug, you trade ready-to-use convenience for a powered applicator. **Pros:** - Battery-powered wand, no pumping or hose needed - 1.3 gallon concentrate mixed 1:1 with water - Largest working volume in this set - Maker states lawn and plant safety when used as directed **Cons:** - Only two specifications published - Requires mixing before every use - 11.5 lb shipped, heaviest here to carry and store - Premium price position despite being a concentrate [Check Price on Amazon](https://www.amazon.com/dp/B0F1W5W7RZ?tag=ijcci-20) ## 6\. DeckMax Premium Deck Cleaner Kit — Best premium **The only kit here that includes a scrub brush, and the only one naming Trex and Timbertech by brand.** [![DeckMax premium deck cleaner kit with clear concentrate and natural bristle brush — the only pick covering up to 1,500 sq ft](https://m.media-amazon.com/images/I/41N3L27fvFL._SL500_.jpg)](https://www.amazon.com/dp/B085P25DX7?tag=ijcci-20) **Best for:** Trex and Timbertech owners with a large deck **Skip it if:** you have no interest in scrubbing Coverage is where this one separates itself completely. DeckMax states up to 1,500 square feet from a 64 fl oz concentrate, which is roughly eight to twenty-five times what Wet & Forget's 60 to 180 square foot band delivers from the same bottle size. That's the trade you're making at the most expensive price position here: dilution work in exchange for area. It's also the only pick that names composite brands directly. DeckMax says the formula is safe for Trex, Timbertech and wood decking, and that it targets environmental black and green staining on both composite and natural boards. The kit includes a natural bristle brush, which the company notes competitors don't supply, and the concentrate works either by hand or through a pressure washer. The formula is described as zero VOC with no harsh solvents, and it arrives as a clear liquid weighing 4 lb, model WCC. Now the honest part. Including a brush means scrubbing is expected. This is the least passive product in the set, and on a 1,500 square foot deck that's real labour. There's no sprayer, no hose adapter and no warranty published. You're buying coverage and composite specificity, and paying for both in effort. **Pros:** - Coverage stated up to 1,500 sq ft - Natural bristle scrub brush included - Names Trex and Timbertech compatibility - Zero VOC and no harsh solvents per the maker - Works by hand or through a pressure washer **Cons:** - Most expensive pick here - Scrubbing is part of the method - No sprayer or hose adapter supplied - Concentrate must be mixed correctly to hit stated coverage [Check Price on Amazon](https://www.amazon.com/dp/B085P25DX7?tag=ijcci-20) ## Which application method suits your deck? Four methods appear across these six picks, and they're not interchangeable. Pick the wrong one and you'll either exhaust yourself or waste product. - **Trigger sprayer.** Wet & Forget only. Precise, slow, best on decks under a couple of hundred square feet or on spot treatment of returning black patches. - **Hose-end adapter.** Moldex and RMR. Fastest for broad flat areas and the only way to reach siding above the deck without a ladder. You give up control over concentration. - **Battery-powered wand.** The 1.3 gallon 30 SECONDS kit. Reach plus volume with no pumping, at the cost of mixing and 11.5 lb to lug. - **Manual pour and scrub.** DeckMax, with its included brush. Slowest and most physical, and the only one built around a concentrate that stretches to 1,500 sq ft. The Pro 128 fl oz refill sits outside all four, because it's ready-to-use liquid with no applicator listed. You'll need your own pump sprayer for that one, which is worth factoring in before you order. My rule of thumb: if you can see the whole deck from one hose bib, buy a hose-end pick. If mildew keeps returning in the same shaded corner, buy the no-rinse trigger bottle and treat that corner twice a year. ## How much surface does one bottle actually cover? Only two of the six publish a coverage figure, and the gap between them is enormous. Wet & Forget states 60 to 180 square feet per 64 fl oz bottle, and explains the spread: heavily stained and porous surfaces absorb more product. DeckMax states up to 1,500 square feet from the same 64 fl oz, because it's a concentrate you dilute. Those aren't competing claims about the same thing. One is ready-to-use liquid landing on wood, the other is a concentrate before water. For the four with no stated coverage, you're working from volume alone. The Pro refill gives you 128 fl oz. The 1.3 gallon kit gives you roughly double that once mixed 1:1\. Moldex and RMR both start at 64 fl oz but dilute through the hose, so the effective area depends on your water pressure and how long you hold the trigger. Measure your deck before you buy. Length times width, then add fence or siding if you're treating those too. A 12 by 16 foot deck is 192 square feet, which already exceeds a single Wet & Forget bottle at the top of its stated range. Buy two, or buy a concentrate. ## What goes wrong when you stain over mildew? Sealing a live colony under a finish is the single most expensive mistake in deck maintenance, and it's why the best deck stain to prevent mildew conversation has to start here. Three failures follow, in order. First, the black bleeds back through the film within a season, usually along the shaded edge and around fasteners where moisture lingers. Second, you can no longer treat it, because the cleaner can't reach the growth through the coating. Third, your only remaining option is a full strip and re-clean, which costs more in labour than the original wash ever would have. The signal to watch for is grey. Weathered wood turns grey from degraded lignin, and that layer holds organic debris that feeds mildew. Every product here targets the visible black and green staining rather than the grey itself, so a deck that's gone uniformly grey may need more than one pass before it looks clean. Two of these products are explicitly designed for repeated low-effort use: Wet & Forget's no-rinse method and Moldex's spray-and-rinse cycle. Those fit a twice-yearly rhythm better than the scrub-based approach does. ## How often do you need to repeat this? Wet & Forget's own timeline answers this indirectly and it's the most useful number in the whole set. Green staining clears in one to two weeks, black staining can take several months, and long-established staining runs 6 to 12 months. That tells you the growth is persistent, and persistent growth means recurring treatment. Here's the trick with shaded decks. Mildew doesn't return evenly. It comes back first where the boards stay damp longest: north-facing sections, under railings, beneath planters, along the wall where water sheets off siding. Treating those spots twice a season is cheaper and faster than treating the whole deck once a year. That's an argument for keeping a trigger bottle on the shelf rather than only a hose-end unit. Spot treatment wants precision. Annual whole-deck washing wants reach and volume. Watch your weather window too. Wet & Forget asks for a dry surface, a cool day and no rain forecast within 4 to 5 hours. Rinse-based products need less lead time, since the water is part of the process. ## The verdict If your deck sits in shade and mildew comes back every year, buy Wet & Forget Outdoor Cleaner & Stain Remover. The no-rinse method means you can treat the same problem corner in ten minutes twice a season without dragging a hose out, and the bleach-free, non-acidic chemistry is the gentlest published in this set. Accept the timeline: black staining can take several months to clear. If you're doing a single large annual clean across deck, fence and siding, buy the 30 SECONDS 1.3 gallon kit with the battery-powered wand. The powered applicator plus a 1:1 mix ratio gives you the most working volume and the least physical effort of anything here, and it reaches what a trigger bottle can't. The cutoff is 200 square feet of continuous surface. Below that, a 64 fl oz trigger bottle is enough and precision beats reach. Above it, you want a hose-end or a powered wand, because hand-spraying a large deck is a job you'll only do once. Two conditions flip this. Composite decking with a brand name on it points to DeckMax, the only kit naming Trex and Timbertech and the only one covering up to 1,500 sq ft. Tight budget and siding to reach points to Moldex, the cheapest pick, with a claimed 40 ft spray reach. ## FAQ ### Is deck stain to prevent mildew worth buying, or is cleaning enough? Deck stain to prevent mildew is worth buying, but only after a thorough clean. A finish applied over live growth traps it and the black returns through the film within a season. Wash first with a mildew stain remover, let the boards dry fully, then apply your finish. The cleaning step is what determines how long the stain lasts. ### What does the extra money buy in this category? The extra money buys coverage and applicators, not stronger chemistry. DeckMax sits at the top of this range and delivers a stated 1,500 sq ft plus a natural bristle brush from one 64 fl oz concentrate. The cheapest pick, Moldex, gives you a built-in hose-end sprayer and a claimed 40 ft reach. Both work; the premium option asks for scrubbing. ### Are these products safe on composite decking like Trex? Safe on composite decking is claimed by three picks here. DeckMax names Trex and Timbertech directly. RMR lists composite decking, PVC and aluminum among its surfaces. The 30 SECONDS 1.3 gallon kit lists composite and stained surfaces. Always spot-test a hidden board first, since composite formulations vary by manufacturer and production year. ### Wet & Forget or Moldex: which should I choose for my deck? Wet & Forget or Moldex comes down to patience versus speed. Wet & Forget needs no rinsing and uses a bleach-free, non-acidic formula, but black stains may take several months. Moldex contains bleach, attaches to a garden hose with a claimed 40 ft reach, and acts fast. Choose Wet & Forget near garden beds, Moldex for quick results. ### What is the most common mistake with deck mildew treatment? The most common mistake is treating only the visible black patches and ignoring the grey film feeding them. Weathered wood holds organic debris that mildew eats, so a spot treatment clears the symptom and leaves the cause. Wash the full surface, then spot-treat shaded corners twice a season where moisture lingers longest. Measure your deck this weekend before you order anything. That single number decides whether a 64 fl oz trigger bottle is enough or whether you need a concentrate, and it's the step most people skip. Then start with Wet & Forget Outdoor Cleaner & Stain Remover, because a cleaner that needs no rinsing is a cleaner you'll actually use again next spring, and this problem is a repeat visitor rather than a one-time job. The finish you eventually put down will last longer for it, which is the whole point of cleaning first. *Last updated: September 16, 2026\. Product availability re-checked.* *This page contains affiliate links. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you.*