Soft Wash Skid Setup and Tank Mixing
Why this matters
A softwash skid puts a real chemical hazard on the truck, not a cleaning product in the loose sense the word gets used around a shop. Sodium hypochlorite (SH) burns skin and eyes on contact, releases chlorine gas if it meets an acid or ammoniated product, corrodes plumbing it was never plumbed to touch, and bleaches fabric, stains metal, and kills landscaping on drift alone. Get the mix too weak and the mildew that brought the customer to you the first time is visible again in a season, a callback that reads as a bad clean rather than an underdosed chemical. Get it too strong and you have not cleaned harder, you have raised every one of those risks at once for no gain in result. This procedure covers the setup and the math that keep the mix at the strength the job needs and nowhere near the strength that hurts someone.
What a softwash skid actually is, and what it is not
A skid is a dedicated low-pressure chemical delivery system: one or more poly tanks, a low-pressure pump built for chemical service (commonly a 12-volt diaphragm pump), and plumbing sized for that pump's flow, entirely separate from the belt-drive high-pressure washer that handles rinse and surface-cleaner work. See [Downstream versus Upstream Injector Setup] for how chemical gets metered into that plumbing; this article owns what goes in the tank and how it gets there.
Plumbing and fittings on a skid need to be chemical-compatible, not just pressure-rated: poly or chemical-grade PVC for tanks and lines, Viton or EPDM seals rather than a rubber compound the product datasheet does not name, and brass or stainless fittings rather than mild steel or aluminum, which SH corrodes visibly within a season of regular contact. A skid built from parts pulled off a standard high-pressure rig, rather than parts rated for the chemical it will carry, is a leak waiting on a schedule nobody set.
PPE and the mixing environment, before the tank is opened
Eye protection and the glove the product's safety data sheet (SDS) names go on before any container is opened, every time, not just for a fresh batch. Mix outdoors or at an open tailgate with air moving past you, never in a closed van box or a utility room, and confirm nobody, especially a child or a pet, is within reach of the mixing area before you start. And if that does not hold, meaning SH reaches skin or eyes despite PPE, flush the affected area with clean water for at least 15 minutes, the standard flush duration for a corrosive chemical exposure under ANSI Z358.1, while someone else gets medical guidance on the phone, and work does not resume until the exposure has been evaluated.
Mixing order, ratio math, and the target working concentration
Water goes into the tank first, concentrate second, because both acids and strong alkalis like SH release heat diluting into water, and concentrate poured into an empty container followed by water is how a splash reaches a face. Confirm the source concentrate's actual percentage on its own label before you calculate anything: bulk SH supply commonly runs in a range of roughly 10 to 12.5 percent and a bottle that has sat in heat or sunlight loses strength over time, so the number on the label, checked fresh, governs the math below, not a percentage remembered from the last delivery.
General house-wash chemistry commonly targets a working strength in a range of roughly 1 to 3 percent SH at the surface for standard mildew and algae, adjusted upward for a heavy mildew load or a roof application and downward for delicate material, and the product's own label sets the number for the specific product in your tank. This walkthrough uses a concrete starting point of 1 percent to show the math; treat it as a common starting point to tune against the label and the job, not a universal figure.
Worked dilution: source concentrate at 12.5 percent SH (confirmed fresh off the jug label), target working strength 1 percent, tank size 35 gallons total.
- Dilution factor: 12.5 percent divided by 1 percent target equals 12.5, meaning 1 part concentrate to 11.5 parts water, 12.5 total parts.
- Concentrate volume: 35 gallons divided by 12.5 parts equals 2.8 gallons of concentrate.
- Water volume: 35 gallons minus 2.8 gallons equals 32.2 gallons of water.
- Check: 2.8 gallons at 12.5 percent strength, spread across the full 35-gallon batch, gives 2.8 times 12.5 divided by 35, which equals 1.0 percent, matching the target.
A surfactant is typically added after the SH, at whatever small percentage its own label states, to help the mix cling to a vertical surface through dwell time rather than sheeting straight off; add it last so it is not sitting in concentrated chemical any longer than mixing takes.
What changes the target, and what does not
Surface and soil load move the target within the range above, source concentration does not. A weathered cedar fence or vinyl siding with light seasonal mildew sits at the low end of the range; heavy algae buildup or a job that has gone two seasons without service sits at the higher end, still inside the label's stated range rather than past it. Roof work is its own case, and it runs the opposite direction from what intuition suggests: a roof mix commonly runs weaker than a siding mix even though roofing algae (commonly the black-streak organism Gloeocapsa magma) is more established growth than siding mildew, because a roof gets a longer dwell instead of a stronger mix to protect the granules underneath. Roofing also carries its own gate on method and pressure, covered in the [Nozzle Tip Selection by Surface and Task] article, so a roof job's chemistry gets read off the product label for roof application specifically rather than scaled up from the house-wash number by feel.
What never changes the target is a weaker source concentrate. If the jug on hand reads meaningfully under the percentage the last batch used, recalculate the dilution from the actual number rather than adding a little extra concentrate to compensate; the math above exists precisely so a strength change on the supply side does not turn into a guess at the tank.
Never combine chemistries in the same container
SH mixed with an acid, including an oxalic-acid-type rust remover, or with an ammoniated product releases chlorine or chloramine gas, both of which injure the lungs. Rinse any container between different chemistries and never top off a tank that last held a different product without confirming what it actually held. Any sharp or chlorine-heavy odor during mixing is the stop signal: move upwind immediately and do not lean in to identify the smell, because both gases deaden your sense of smell at concentrations that are already harmful before you notice anything wrong. And if that does not hold, meaning someone is already coughing or has eye or throat irritation when the odor is caught, that person leaves the area for fresh air immediately, everyone else stays clear until the vapor has fully dispersed, and the person affected gets medical evaluation rather than waiting to see if it passes.
Drift, runoff, and landscape protection
Assess wind before the first spray, not after overspray has already landed somewhere it should not have. Pre-wet and cover landscaping directly in the application path, hold off application within a distance the wind that day actually allows rather than a fixed number that assumes calm air, and keep application off metal railings, light fixtures, and vehicles you have not been asked to clean, since SH stains and pits bare and painted metal on contact. And if that does not hold, meaning wind shifts mid-job or overspray reaches a surface or plant it should not have, stop application immediately, flush the affected area with fresh water within minutes while the residue is still wet, and document what was affected before you leave the site so the customer hears it from you first, not from the browning shrub a week later.
Worked example: mixing and verifying a tank for a two-story house wash
The job is a standard house wash, moderate mildew, no roof work today, using the skid built above.
Concentrate check first: the fresh jug on hand read 12.3 percent on its label, close enough to the 12.5 percent used in the math above that the same 2.8-gallon concentrate volume was used without recalculating (the two-tenths-of-a-percent difference moves the resulting strength by less than two-hundredths of a percent, well inside what a field test strip can even resolve). Water went into the 35-gallon tank first, filled to the 32.2-gallon mark on the tank's sight gauge, then 2.8 gallons of concentrate measured into a graduated container and added, then the surfactant per its own label added last.
The finished batch was checked with a chlorine test strip before it went anywhere near the house: the strip read within the expected band for a 1 percent working solution. Application ran per the skid's metering setup, held off two flowering shrubs directly under the eave by pre-wetting them and working around rather than through that section, and finished with a fresh-water rinse of the same shrubs immediately after the house rinse was complete, as a standing precaution rather than a response to any actual drift that day.
Verify before you spray
- Confirm the source concentrate's percentage on its own label, checked fresh, before calculating any dilution.
- Mix water first, concentrate second, PPE on before the first container opens.
- Check the finished batch with a test strip against the expected band for the target working strength before it leaves the tank.
- Confirm wind direction and cover or hold off on anything in the drift path before the first spray.
- Have a fresh-water rinse ready for landscaping and any accidental overspray before you start, not after.
References
- See related: Downstream versus Upstream Injector Setup
- See related: Nozzle Tip Selection by Surface and Task
- ANSI Z358.1 for emergency eyewash and shower flush duration on a corrosive chemical exposure
- Manufacturer SDS and product label for the specific SH concentrate and surfactant in use, for exact PPE, concentration, and compatible material requirements