well,
The active ingredient in Wet & Forget Outdoor is alkyl dimethyl benzyl ammonium chloride. This ingredient is present in many common household sprays and in anti-bacterial wipes. Wet & Forget Outdoor doesn't contain harsh chemicals such as bleach, ammonia, sodium hydroxide, lye, or trisodium phosphate.
alkyl dimethyl benzyl ammonium chloride is expensive from the few links I clicked, $175 for 100 grams, it's like a lab product.. The concentrated wet&forget is %9.9% alkyl dimethyl benzyl ammonium chloride, maybe that justifies the price of w&f but I'm sure if can find this chemical in bulk is worth it, and %10 is an easy ratio to measure or just eyeball it (unlike something like %0.5) and not worry about putting too much or too little mixed with water.
TSP is a great product for cleaning greasy ceilings over stoves when repainting kitchens etc so that new paint sticks good, even a good primer over oily walls/ceiling won't work. Many painters prep by scrubbing all the walls with a TSP solution hence it's a main marketing point on the box but nowadays paint and primer in-1 usually works fine. I scrubbed older baseboards with TSP so the baseboards don't look like they have no groves and been painted 50 times but I mostly give up on those older baseboards that don't have to be perfect for a recoat.
TSP can be harsh if not diluted enough it can remove paint, so I'd be very hesitant to spray it on a house or even vinyl siding which is basically powdered colorants built into the plastic. TSP is actually added to cereals etc in small amounts for whatever reason despite the warning and that now they have a phosphate-free version was all they had in the store so I stopped buying it for painting I'd just use a regular kitchen degreaser, light sand, prime and then topcoat if needed.
Anyway, I'm getting into learning softwash power washing as to not potentially damage siding using just water power washer pressure which works great and I always prefer a no chemical approach but can be risky esp for non vinyl areas. Just regular 'siding soap' with a residential grade power washer and a black low-pressure soap tip should be okay for most things but you see these foam cannon low pressure washes take 2 seconds and barely need any pressure and it removes algae and mildew amazingly and shouldn't harm surfaces. Im sure there's a lot that can go into what mixes people use who really know what they're doing, different mixes for roofs, vinyl, wood, masonry etc. And the key is that some prolong the affect so it doesn't get mildew again right away. Some are doing soft wash with just $2.50 gallons of Walmart pool bleach or something but I don't know if that prevents further mildew like w&f or other mixes. But @ $40 a gallon of w&f concentrate to cover only about 2k sq ft can add up in cost after pressure washing the whole property but it really does work well and last many years if it's not too much of a shady north-facing algae-prone environment.
W&F claims here
Wet & Forget | Complete Line of No Scrub, Bleach-Free Products
that alkyl dimethyl benzyl ammonium chloride is only the surfactant (to make it stick and prolong cleaning affects, so maybe it's naturally also a surfactant and no need to add something else to make it last longer asif maybe non ionic surfactant is safe for building materials and would work for this application), but it claims Water as the Solvent.
Have a lot to research on soft washing but W&F is definitely a good product and there's tons of power washing to be done.
