Quick answer
Wet & Forget works, but slowly: its listings say green stains clear in 1 to 2 weeks and black or long-established stains can take several months to a year, with no scrubbing and no rinsing. For most homes the one to buy is the Wet & Forget Outdoor Cleaner Concentrate, 64 oz, rated 4.3 stars across 5,690 Amazon ratings, which dilutes 1 to 5 and treats 375 to 1,125 sq ft. Choose the Xtreme Reach Hose End for roofs and second-story siding. It is for surface growth outdoors; mold inside walls or on wet drywall needs the moisture fixed and the material cleaned or removed, not a spray.
Buyer's Guide
What to know before you buy.
What Wet & Forget Is, and How It Works Without Scrubbing
Wet & Forget is a leave-on outdoor cleaner. Its listings describe one method for every format: wet a dry surface, do not rinse, and let wind and rain carry the dead growth away over the following weeks. The claim is stain removal for moss, mold, mildew and algae on hard outdoor surfaces, and the named list runs from roofs and siding to decks, driveways, canvas and aluminum. Each listing describes the formula as bleach free and non-acidic, and none of them names the active ingredient, so the label on the bottle is where to look for that. This page compares the concentrate, the hose-end bottle, the ready-to-use spray and the shower cleaner; the comparison is built from their listings, published directions and buyer ratings, not from hands-on use. What follows is how to pick a format, what the timeline really means, where the cautions apply, and the mold problems the brand is not built for.
Concentrate, Hose End or Ready-to-Use: Pick by Area and Height
Pick the format by how much surface you have and how high it is. The 64 oz concentrate dilutes 1 to 5 in a garden sprayer and is stated to treat 375 to 1,125 sq ft, which suits siding you can reach from the ground, a deck or a patio. The hose-end bottle treats up to 2,000 sq ft and its jet tip is stated to reach 30 ft, so it is the one for a roof or a gable wall, and its fan tip handles a driveway. The ready-to-use trigger bottle covers 60 to 180 sq ft, which is porch steps, a planter or a fountain. Do the arithmetic before buying: a 10 by 20 ft driveway is 200 sq ft, one wall of a 40 ft ranch house 8 ft tall is 320 sq ft, and the listings warn that heavily stained and porous surfaces absorb more. Note the temperature lines. The hose end wants above 55°F; the ready-to-use bottle and the 128 oz concentrate say above 32°F.
The Timeline: Green in Weeks, Black in Months
Set your expectations from the listing, not the name. Every Wet & Forget outdoor listing gives the same schedule: green stains clean up within 1 to 2 weeks, black or heavy stains take several months, and long-established stains can take 6 to 12 months. Treated surfaces are stated to stay clean up to 1 year or longer. In practice that means green algae on north-facing vinyl siding in spring is a good match, while a black-streaked roof treated in October may still look streaked at Thanksgiving. The weather also has to cooperate: the listings ask for a dry surface, a cool day, no rain for 4 to 5 hours after spraying, and no rinsing at all, because rinsing removes the product that is meant to keep working. If a surface has to be clean for a date on the calendar, this is the wrong product, and a spray-and-rinse cleaner such as 30 SECONDS, or a pressure washer on a surface that can take one, is the right one.
Plants, Ponds, Pets and Roofs: The Cautions on the Label
The cautions are short and they repeat on every outdoor listing. Flush plants with water before and after application, and keep the spray 100 ft away from lakes, ponds, rivers and streams. With a hose-end jet reaching 30 ft, overspray and runoff travel, so wet down the beds under a wall before you start and again when you finish, and do not treat a surface that drains straight into a pond. The listings read for this page say nothing about pets or children, so keep both off a treated deck or walk until it is dry and follow the container label. On roofs, apply from the ground or a ladder with the hose-end jet rather than walking a wet roof, and if a downspout feeds a rain barrel, disconnect it until the first rain has washed the roof. Rain within 4 to 5 hours washes the product off before it has worked, and the concentrate listing says unused diluted solution can be stored in the sprayer for next time.
Who Should Not Buy Wet & Forget: Mold Inside the House
Wet & Forget is a stain remover for outdoor surfaces, and it is the wrong purchase for the mold problems that bring most readers to this site. Growth on drywall, inside a wall, on a damp basement wall or on a ceiling after a leak is a moisture problem first, and EPA puts it plainly: the key to mold control is moisture control. EPA's guidance is to scrub mold off hard surfaces with detergent and water and dry them completely, to dry wet materials within 24 to 48 hours of a leak, and to throw away absorbent materials such as ceiling tiles and carpet if they become moldy. A leave-on outdoor spray does none of that, and the shower cleaner's listing claims soap scum and grime, not mold. For a patch larger than about 10 sq ft, or any growth that followed a flood or a sewage backup, EPA points to its professional remediation guidance, and that is when a mold remediation company, not a bottle, is the right call.
Keeping Outdoor Growth From Coming Back
Green and black growth outdoors is a shade-and-moisture problem, and the listings' up to 1 year claim lasts longest on a surface that dries. Trim shrubs back from siding so air moves behind them, clean the gutters so they do not overflow down a wall, and extend downspouts so water does not sit at the foundation; EPA lists regular gutter cleaning and ground that slopes away from the house among its moisture-control tips. A north wall under a tree will always green up faster than a south wall in the sun, so plan on treating it each spring. Sweep decks so leaves do not hold water on the boards. Indoors, the shower cleaner's weekly routine handles soap scum, but mildew on grout needs the bathroom fan run during and after every shower, which EPA lists among the ways to reduce humidity, and a hygrometer reading under 60 percent in the rest of the house.
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