So you want to know what goes onto that tank
Most people buying a pressure washer stop at GPM and PSI and think they understand the machine. They don't. The real variable is chemistry, and getting it wrong will either destroy the surface you're cleaning or leave you spending three hours scrubbing something that should have come clean in twenty minutes. I spent six years doing commercial exterior work before I figured out how to read a label without guessing. The industry doesn't help you learn this. Contractors get handed a bottle of orange surfactant and told to spray it on brick. That's how people end up with stained siding and ruined mortar joints.
Pressure Washing Chemical Guide
Here's the practical breakdown of what each class of chemical actually does, what it does poorly, and where it fails entirely. Surfactants are the workhorse and the most misunderstood product in the tank. A surfactant lowers surface tension so water can wet a surface it would otherwise bead off of. On a vinyl siding job, a good all-purpose surfactant diluted at roughly 1 part to 20 parts water will cut dirt, allergens, and mild surface grime. You spray it on, let it dwell for five to eight minutes, then rinse. That's it. But here's what nobody tells you: if the surfactant dries on the surface before you rinse, it leaves streaks that look exactly like permanent stains. I once watched a crew leader let a foam cannon charge dry on south-facing siding in direct August sun. He came back forty minutes later and found orange-tinged sheets. He ended up having to strip and repaint three stories because he couldn't get the surfactant residue out. Always keep the surface wet until you rinse. Hydroxide-based cleaners (like sodium hydroxide, commonly sold as "brightener" or alkaline cleaner) break down organic matter. Oil, grease, tree sap, and biological growth. This is what you reach for on a driveway that's been sitting under a leaking garage door or around a grill station. NaOH at 5 to 10 percent in your tank will eat through years of embedded oil. But it's caustic. It will burn skin, damage aluminum siding if left too long, and turn wood gray if it penetrates the grain. I've seen house wraps degraded because someone sprayed hydroxide cleaner from too close and the solution wick-migrated behind the material. Keep your tip at least eighteen inches from the surface when using anything stronger than plain surfactant.
Oxalic acid and other acid-based cleaners handle inorganic deposits. Hard water stains, efflorescence on brick, rust streaks, mineral buildup from well water. Oxalic acid is the one most people should have in their rotation. At 10 to 15 percent concentration, it dissolves iron stains and calcium without being as aggressive as muriatic acid. I ran into a situation last year on a restoration job where a homeowner had tried using muriatic acid on a limestone facade and burned through three inches of soft stone. Oxalic would have done the same stain removal with a fraction of the damage risk. Never use muriatic acid on anything other than bare concrete, and even then dilute it to no stronger than 10 percent. The fumes alone will set off respiratory issues for anyone standing nearby, and the splash-back risk is real. Bleach solutions (sodium hypochlorite) kill mold, mildew, algae, and some bacteria. They do not remove the biomass that's already attached. Bleach kills the organism but leaves the black or green residue behind, which is why you still see supposedly "cleaned" houses looking spotty. For biological growth, you need a detergent or surfactant to actually lift the material off the surface. Bleach is a pretreatment or a disinfectant step, not a cleaning agent. Mix at 1 part household bleach to 3 parts water for light mold, or up to 1 part bleach to 1 part water for heavy growth on durable surfaces like concrete. Never mix bleach with acid-based cleaners. That creates chlorine gas, which is dangerous enough that OSHA has specific regulations around it. Store them in separate tanks, period. Dedicated moss and algae removers often contain quaternary ammonium compounds or specialized surfactant blends. They work slower than bleach but leave less residue and are safer for surrounding landscaping. If you're cleaning a deck near flower beds, I'd rather you use a quats-based product than splash bleach everywhere and kill half your garden.
Get the Full Details
The dilution ratios I just gave are starting points. The actual concentration you need depends on water hardness, ambient temperature, surface porosity, and how long the material has been sitting there. A stain from two years ago needs more chemical and more dwell time than one from last week. That's just basic chemistry, nothing fancy. Here's a mistake I see constantly: people pre-mix their entire season's worth of chemical solution in a five-gallon bucket and store it. Sodium hypochlorite degrades over time, especially in warm conditions. After a few weeks the solution loses strength and you end up using more product for less effect. Make fresh batches weekly, and keep them in a cool shaded area. Another thing people get wrong is skip-rinsing. You apply chemical, let it dwell, then rinse. Don't just let it air dry and hope the rain washes it off. Dwell times are measured in minutes, not days. If a product says five to ten minutes, rinsing after twenty is way too late and you risk chemical etching or residue staining. I learned this the hard way on a stucco job in Phoenix where the ambient temperature was over 100 degrees. The solution dried in under three minutes, and the stucco came out with chalky white patches. Had to do a full acid wash to restore it.
Equipment matters too. A soap nozzle (black tip, usually 40 degrees) on your wand keeps the chemical on the surface longer by applying it gently. Switch to a white or green tip for rinsing. Don't try to blast chemical off with a high-pressure tip — you'll just drive it deeper into porous materials. Landscaping protection is another area where people shortcut things. Wet down plants before you start, cover sensitive shrubs with plastic, and rinse them thoroughly when you're done. Even diluted detergent can damage foliage. I had a client fire me once — unfairly, I thought — because a row of boxwoods on the property line turned brown after we washed the house. The issue was overspray drift, and I should have used a lower pressure setting closer to the planting bed. Lesson learned, but it cost me the job. If you want a quick reference, here's what I carry in my truck on any given day: one gallon of all-purpose surfactant, one quart of oxalic acid powder, one quart of sodium hydroxide crystals, one gallon of bleach, and a couple of gallons of neutral pH cleaner for delicate surfaces like painted wood or composite decking. That covers probably 90 percent of residential jobs. The other 10 percent usually involves something I haven't run into yet, at which point I call a supplier and figure it out.
The bottom line is this: chemistry is a tool, not a magic wand. Pick the right product for the contaminant, dilute appropriately, respect dwell times, and protect what you don't intend to clean. Get those four things right and your pressure washing results will be consistent. Get them wrong and you'll spend more time fixing mistakes than actually washing.
