What kind of siding. I suspect the answer might change for different siding

 

Depends on how dirty it is. Depends on what you might kill with overspray. Depends on the injector ratio on your pressure washer (if you're using a pressure washer). Depends how much you want to spend. Depends what chemicals you have access to. Depends how old the chemicals are. Depends on the temperature.

My basic mix for vinyl is 2 gallons of fresh 12.5% sodium hypochlorite with 2 gallons of water and 5 ounces of a commercial house wash surfactant called Elemonator. Then I tweak it from there.

 

I normally mix equal parts of bleach and water. I'll add TSP if there is stubborn grime. Here in the southeast mildew is the main culprit. Bleach can kill vegetation but wetting it good with water before, during and after usually prevents damage. I like bleach because it's cheap and effective but the commercial cleaners are more diy/environmentally friendly.

I prefer to apply the cleaner with a pump up garden sprayer. It gives better control and you don't have to run the chemicals thru the pressure washer.

 

I usually dilute some Krud Kutter and spray it on the siding and let it marinate for up to 20 minutes or so. Then I pressure wash it all off. Many times I just use my PW without any cleaner so I can avoid leaving any kind of detergent residue before painting.

If you've ever seen someone pressure wash with warm/hot water at low pressure (called down streaming I think) it does a wonderful job without the dangers of high pressure pushing water up under the siding or blasting around the edges of windows and doors, etc.

 

Down streaming just means you are adding the chemicals after the pump. This extends the life of the pump, but limits your chemical ratio - a lot of water has to go through to siphon up a little chemical. It's good enough for siding, but you can't wash a roof like that. Temperature is a separate issue. A few guys use hot water on houses, but it's mostly used on really greasy, disgusing stuff like gas stations and dumpster pads.

 

I believe it's Masonite siding and I am just trying to clean the house. I had it painted a couple years ago and it needs a cleanup


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You have to be careful with masonite. If it gets too wet it will swell and eventually fail apart. Don't go crazy with a pressure washer! As long as it stays well painted it can last a long time.

Just what needs to be washed off? mildew? dirt/dust?

 

Mostly dusty & dirty


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If there isn't any mildew you may not need bleach. TSP is a great detergent for washing siding although laundry detergent should work ok. I'd mix up the cleaner, spray it on with a pump up garden sprayer, let it set/work but not dry and then rinse with a pressure washer. A garden hose can be substituted for the PWer.