Enter surface, chemistry, and ambient conditions — get the exact safe dwell window. Substrate damage warnings and surfactant guidance included.
Laminated-style 2-page field reference: 12 surfaces × 5 chemistries dwell grid, SDS quick-reference for 5 core chemicals, substrate damage warning matrix. Pocket-sized (8.5"×11" folds to 5×4.25").
Check your inbox — the 2-page PDF is attached. Print it, laminate it, truck it.
Every soft-wash contractor knows SH concentration matters. What gets less attention is dwell time — the window between chemical application and rinse during which the active chemistry does its work. Get it wrong in either direction and you lose: too short and the organic growth isn't killed; too long and the substrate pays the price.
The problem is that most "dwell time guides" are static reference charts built for a single ambient condition — typically 70°F, partial sun, moderate growth. On a real job, those conditions vary constantly. A full-sun concrete pour at 92°F surface temperature has a fundamentally different safe dwell window than the same job in morning shade at 72°F.
Four environmental variables have the biggest impact on usable dwell time:
Most substrate damage discussions focus on PSI. But chemical damage — specifically SH damage on sensitive substrates — is more common and less visible until it's too late. Powder-coated aluminum exposed to SH above 1% for more than 6 minutes can begin to delaminate at the microscopic level; the chalking or blistering appears days later, after you've been paid and left the site. EIFS (Dryvit, synthetic stucco) is even more sensitive — the thin acrylic finish coat can crack or blister at SH concentrations that would be fine on painted aluminum.
Limestone and marble represent a categorically different risk: calcium carbonate substrates are chemically incompatible with acid cleaners. Any acid contact — even highly diluted — causes irreversible etching. This calculator flags this as unsafe before you proceed, because the cost of a mistake on a limestone fireplace surround or marble entryway is not recoverable.
The dwell-time engine uses a per-substrate base window derived from industry field data, then applies multiplicative adjustment factors for temperature, humidity, sun exposure, surfactant use, and organic growth severity. The result is a min–max window in minutes that reflects real ambient conditions, not laboratory benchmarks.
Compatibility warnings fire before the dwell output when a chemistry/substrate combination is unsafe — acid on limestone, SH above substrate maximum on powder coat or EIFS. These are hard stops, not suggestions.
The SH Mix Calculator handles the upstream question — how many gallons of SH and water to mix. This calculator handles the downstream question — how long to leave it on. Use both on every soft-wash job.
The downloadable Pocket Card compresses the full dwell-time matrix into a laminated-format 2-page reference: 12 substrates × 5 chemistries at ambient conditions, SDS quick-reference for the 5 core pressure washing chemicals (SH, sodium percarbonate, oxalic acid, citric acid, ammonium chloride), and the substrate damage warning matrix. Sized to fold into a shirt pocket or clip to a rig clipboard.
For operators running OSHA-compliant chemical programs, the full Chemical Safety Deep Dive goes deeper: SDS binder templates, PPE log by chemical class, mixing hazard matrix, and NPDES chemical flagging for stormwater compliance. The pocket card is a field tool; the Deep Dive is the documentation behind it.