Soft-Washing Technique by Siding Material

Why this matters

The Soft Wash House Procedure standard sets one mix ratio and one pressure ceiling because most vinyl siding tolerates a wide margin either way. That margin does not carry across materials: the same mix that leaves vinyl untouched can strip a bare cedar shake's finish, and the same pressure ceiling that is conservative on vinyl is still too high for a wall built on foam insulation board. Getting the material wrong is not a cosmetic miss, it is either the wash not working (growth returns in weeks) or the wash causing damage the wash was hired to prevent.

The one variable that actually changes: porosity and pressure tolerance

Two properties decide almost everything below: how porous the surface is, and what sits behind it. Neither one is guessable from the color or the age of the siding alone; two houses with the same vinyl on the front elevation can carry a completely different material on an addition or an accent wall, which is exactly why the walkthrough later in this article treats each elevation as its own read rather than assuming the whole house matches the first wall checked. A porous, absorbent material (bare wood, unsealed masonry, stucco) pulls the mix in deeper and holds it longer, which usually means a shorter dwell rather than a longer one, since the goal is contact time on the surface growth, not soaking the substrate. What sits behind the surface decides the pressure ceiling independent of the material's own toughness: a rigid, solid-backed material like brick over CMU tolerates more incidental pressure than a lightweight assembly with an air gap or foam substrate behind a thin finish coat, because pressure that would just bounce off solid masonry can drive water past a thin finish and into the assembly behind it.

Vinyl and fiber cement: the forgiving pair

Both hold up to the standard mix and pressure ceiling with little adjustment. Vinyl's factory finish is close to non-porous, so the mix sits on the surface rather than soaking in, and the standard 5 to 15 minute dwell window and roughly 0.6 percent starting concentration from the wash standard apply without modification. Factory-primed and painted fiber cement behaves similarly once the finish is intact, but check the caulked seams and butt joints before starting: a caulk line that has failed or shrunk back lets the mix track behind the panel at that joint even at soft-wash pressure, so treat any visibly open seam as a spot to keep the wand angle parallel to the wall rather than angling into the gap, and flag it on the ticket as a seam needing caulk repair regardless of how the wash turns out.

Wood: painted, stained, and bare cedar shake

Painted or solid-stained wood tolerates the standard mix but drop dwell toward the short end of the window, 5 to 8 minutes, and rinse promptly; wood's grain holds moisture longer than vinyl, and a full 15-minute dwell on wood risks driving water into open grain at any nick or gap in the finish. Bare or lightly stained cedar shake is the material to treat most conservatively: sodium hypochlorite bleaches raw wood tannins unevenly, and the same mix that reads as a clean kill on painted siding can leave a bare shake roof or wall visibly lighter in the areas that got the most contact time. Shift the ratio toward the lean end (1 to 20 or leaner) and hold dwell to the short end, checking a small test area first rather than committing to a full elevation on an unfamiliar bare-wood finish.

Stucco and EIFS: where pressure becomes the real risk

True stucco (a cement-based coating over a masonry or lath substrate) is porous enough that it needs a thorough pre-wet to keep it from pulling the mix in unevenly, which shows up later as blotchy-looking clean spots once it dries; extend the pre-wet soak beyond the standard's landscaping soak to include the wall itself on a hot day. Pressure tolerance is still governed by the standard's under-100-psi ceiling, applied strictly, since even soft-wash pressure held too close to the surface for too long can push moisture behind an aged or hairline-cracked stucco coat.

EIFS looks like stucco but is a different assembly entirely: a foam insulation board under a thin synthetic finish coat, with a drainage plane behind it that was never designed to shed water forced in from the outside. Many EIFS manufacturers restrict or outright prohibit pressure washing the system for exactly that reason, since water reaching the foam substrate does not evaporate the way it does behind masonry, and moisture trapped there causes mold and rot inside the wall assembly with no visible sign until the damage is advanced. Confirm which system is actually on the wall before treating it as stucco (see the verification section below), and where it is EIFS, route to the system manufacturer's own written cleaning guidance rather than defaulting to this article's stucco approach; that guidance, not a general pressure-washing standard, is what a warranty claim gets measured against.

Brick and masonry joints

Modern hard-fired brick over CMU is the most pressure-tolerant material in this list, but the mortar joints are not: they are more porous than the brick itself and hold the mix longer, which can show up as a lighter halo around each joint if dwell runs long on a hot day, and can also draw dissolved salts to the surface as it dries, a white powdery bloom called efflorescence that is a drying artifact rather than damage but still worth flagging on the ticket so it is not mistaken for residue on the callback. Historic or reclaimed brick, and any soft, underfired brick common in older construction, is the exception to "brick is forgiving": it can spall (flake at the face) under pressure that hard-fired brick shrugs off, so treat an older or unfamiliar masonry building with the same conservative pressure discipline as EIFS until you have confirmed the brick's condition on a small test area.

Aluminum siding and trim

Aluminum tolerates the standard mix and pressure well when the finish is sound, but oxidized or heavily chalked aluminum needs a different read before you start, not a different chemical: the wash can reveal pre-existing unevenness in the coating rather than causing new damage, and mistaking one for the other is common enough to have its own article. See the Troubleshooting sibling on oxidized aluminum looking worse after a wash for the full diagnosis and the pre-job documentation that prevents the callback.

What changes the call: coating condition, not just material type

Everything above assumes a sound, intact finish, and that assumption is the one worth checking before defaulting to a material's usual treatment. A failing paint film changes the read regardless of what is underneath it: chalking, cracking, or peeling paint on wood or fiber cement means water and chemical are reaching bare or under-prepared substrate through the gaps in the film exactly the way they would on a fully bare surface, so a "painted, tolerant" material with a failing coat gets the conservative dwell and ratio this article assigns to bare wood, not the standard treatment for its material class. The same logic runs the other direction: a newly repainted or newly sealed surface on an otherwise porous material, fresh masonry sealer on brick, for example, can tolerate closer to the standard treatment than the bare version of that material would, because the sealer is doing the porosity work the bare surface can't. Read the finish, not just the substrate, before picking a ratio.

One job, three materials: a worked walkthrough

A single-story house: vinyl lap siding on the main body, a stone-look EIFS accent wall framing the entry, and aluminum fascia and gutters running the full perimeter. The crew mixed the standard ratio, 1 part 12.5 percent SH to 20 parts water, about 0.6 percent at the surface, for the vinyl body and ran the standard 5 to 15 minute dwell there without issue.

Before touching the entry wall, the lead technician ran the water-drop test described below and confirmed it was EIFS, not true stucco. The system's manufacturer sticker, found on an access panel near the meter, listed a cleaning restriction: rinse only, no pressure application beyond a garden hose. The crew hand-washed that wall with a soft brush and the diluted mix at hose pressure only, logged the restriction on the ticket, and photographed the manufacturer sticker as the basis for the deviation from the standard soft-wash pressure.

The aluminum fascia and gutters were sound, unoxidized, and took the standard mix and pressure without modification, rinsed clean in the same pass as the adjoining vinyl. The job closed with three technique notes on one ticket rather than one blanket note, which is what let the next technician who serviced that address in a following year know exactly why the entry wall was handled differently without having to ask.

Verify you identified the material correctly before you mix

Two quick field tests settle the calls that matter most before chemical goes on the wall:

• Water-drop absorption test, stucco versus EIFS. Place a few drops of water on an inconspicuous area of the finish coat. True cement stucco absorbs the water into the surface within a minute or two; EIFS's synthetic finish coat is far less absorbent and the water tends to bead or sit longer. This is not a substitute for finding a manufacturer label if one exists, but it catches the case where no label is visible and the wall looks like stucco from the ground.
• Magnet test, aluminum versus steel gutters and trim. A magnet will not stick to aluminum and will stick to steel. This matters because the standard mix and pressure that suit aluminum can leave a bare or thin-coated steel gutter more exposed to rust at any scratch the wash opens up; steel trim gets the same conservative, shorter-dwell treatment as bare wood until the finish is confirmed sound.

References

• See related: Soft Wash House Procedure Standard, Siding Material Compatibility for Soft Washing, Oxidized Aluminum Siding Looks Worse After a Wash.