Wood Species and Treatment Compatibility Reference

Why this matters

The Wood-Safe Low-Pressure Wash Standard and the Wood Brightening and Neutralizing Procedure both state one baseline ratio and lean it "for bare, unsealed, or tropical hardwood sections" without spelling out what that means board by board. Guessing the lean on an unfamiliar species runs in one of two costly directions: too strong a mix bleaches or streaks a species that did not need the extra strength, too weak leaves a heavily tannin-stained hardwood looking barely touched after a full dwell cycle, reading to the customer as a wash that did not work. This reference is the table both procedures call, covering chemistry only, mechanical pressure and technique are owned by the Pressure Setting Reference for Wood Surfaces and the Deck and Fence Cleaning Technique by Wood Type article.

What this reference covers

Two separate chemistries, used in sequence but chosen independently: the sodium hypochlorite (SH) wash mix that removes dirt, mildew, and algae, and the oxalic-acid brightener that lifts UV-grayed fiber and tannin or iron staining afterward. A species can need a lean on one and the standard rate on the other; the table below states both.

Reading the table wrong in one direction costs more than the other. Over-leaning a species that would have tolerated the standard rate mostly costs time, a longer dwell or a second application to hit the same result the standard rate would have reached in one pass. Under-leaning a species that needed the weaker mix is the direction that damages wood: a standard-rate SH mix held on bare cedar for a full dwell window can bleach the surface unevenly in a way no amount of rinsing corrects, since the reaction has already happened by the time anyone notices the color shift. When a species is genuinely unfamiliar and not listed here, the safer default is the lean ratio with a mandatory test patch, never the standard rate on the assumption that "most wood is fine at the standard mix."

Reading tannin and iron staining before you choose a ratio

Tannin-rich species, cedar, redwood, and most tropical hardwoods, carry natural tannins that react with iron to produce a dark, often streaky discoloration wherever a steel fastener, a rusting bracket, or even runoff from an adjacent metal surface has contacted the wood over time. This staining looks similar to ordinary dirt or mildew from a distance and is commonly misdiagnosed as a wash-strength problem, when it is actually a chemistry-mismatch problem: the standard SH wash mix does little to a tannin-iron stain, since that reaction is a different chemical process than the mildew and dirt the wash mix is built to remove, while an oxalic-acid brightener is specifically effective against it, since oxalic acid is a chelating agent that binds and lifts iron compounds. A tech who reads a dark streak radiating from a fastener head as "the wash needs to be stronger" and increases the SH concentration is solving the wrong problem; the same streak responds to the brightening step that was likely scheduled anyway, and calling this out to the customer at the walkthrough, rather than after a wash pass fails to move it, sets the right expectation before the mix is even built.

Worked example: an ipe deck with iron staining around every fastener

A ground-level ipe deck, roughly 8 years old, moderate gray weathering overall plus dark, roughly 2-inch streaks radiating downward from every visible screw head, a pattern covering perhaps a tenth of the total board surface concentrated at the fastener rows.

Reading the table: ipe is tropical hardwood, so the wash mix leans to 1:24 (about 0.5 percent available chlorine from a 12.5 percent source) with a short dwell and a mandatory test patch before full application, per the Wood-Safe Low-Pressure Wash Standard. The wash ran per that standard and cleared the general gray weathering evenly, exactly as expected for a species that tolerates the standard mechanical ceiling; the fastener streaking, as predicted by the tannin-iron read above, did not move under the wash mix at all, confirming it as staining rather than surface dirt.

The brightening pass followed the next day. The table's oxalic response note for tropical hardwood, slower to react, do not over-concentrate to compensate, meant setting the expectation of a longer watch period rather than a stronger mix: the label-rate mix was applied and agitated directly into each fastener streak with the stiff brush called for in the Wood Brightening and Neutralizing Procedure, and the streaks visibly lightened over roughly 18 minutes of dwell, toward the longer end of that procedure's 10 to 20 minute window, compared with the 12 minutes the surrounding weathered field needed to reach even color. Rinsed clear at the standard wood-safe pressure ceiling, the deck came up with the fastener streaking reduced to a faint shadow rather than fully invisible.

That faint remaining shadow is the expected outcome on a deep, years-old iron stain, not a failed brightening pass: a single application lifts most but not always all of a stain that has had years to set into the wood's tannins, and the ticket noted the residual shadow with a recommendation for a second brightening visit in the future if the customer wants it fully gone, rather than re-treating immediately at a stronger concentration that the table's own caution warns against.

The failure mode this example is built to prevent: a tech who does not read the tannin-iron pattern before starting sees streaking that the wash pass barely touched, concludes the wash mix was too weak, and doubles the SH concentration on a second wash attempt. On a tropical hardwood already near the lean end of this table, that stronger mix does not touch an iron stain any more effectively, since SH is not the active chemistry against that specific staining, but it does risk uneven bleaching on the surrounding sound wood that the correctly-diluted first pass left untouched. The deck ends up with a new problem, an unevenly bleached field, layered on top of the original one, the fastener streaking, which the wrong chemistry was never going to solve regardless of concentration.

What changes the answer

A species reading standard on this table can still need the lean treatment if it is freshly milled (under about 6 months from installation), carries a heavy prior sealer or oil finish that slows chemical penetration, or shows any of the coating conditions covered in Removing Old Coating Residue Before a Wash. Conversely, a naturally lean-rated species that has weathered bare and untreated for many years can sometimes tolerate closer to the standard rate, since years of UV exposure have already broken down much of the surface tannin that made the fresh version reactive; this reference's ratios are starting points confirmed by the mandatory test patch, not fixed values to apply blind regardless of the specific board's history.

A mixed-age deck, some original boards and some replacement boards from a repair, is the case most likely to defeat a single test patch entirely, since a patch run on one board population says nothing about the other. Where a property's file notes a prior board replacement, run a second test patch on a known replacement board before committing the whole deck to one ratio, the same discipline the Wood Brightening and Neutralizing Procedure now calls for after the blotchy-result case in this library's troubleshooting set. Treat the two populations as two separate species entries against this table even when both are nominally the same wood, since age and prior exposure, not just species, are what this table's ratios are actually keyed to.

Confirming species before you commit to a ratio

Three quick field cues settle most of the calls this table depends on getting right, faster than guessing from color alone, which fades and grays similarly across several species once weathered. Pressure-treated pine shows a faint greenish or brownish cast in a fresh cut and typically carries visible pressure-treatment tags or stamps near the ends of structural members. Cedar and redwood both show a reddish-brown heartwood tone in a fresh scratch even when the weathered surface has gone gray, and cedar in particular carries a distinct aromatic smell when cut or sanded that redwood lacks. Tropical hardwoods run noticeably denser and heavier for their size than any of the above, dense enough that a fresh cut often will not float, and commonly show an oily sheen on a freshly exposed surface that domestic species do not. Where none of these cues resolve it confidently, treat the board as unknown and apply the lean ratio with a mandatory test patch rather than guessing toward the standard rate.

How to verify you chose correctly

Before committing to a full run at any ratio in this table, confirm the test patch's reaction against the "oxalic brightening response" column's stated character, an even lightening for standard species, a slower and possibly two-stage lightening for tropical hardwood, rather than judging only by how much time has passed. Where a stain does not match the table's tannin-iron description, does not radiate from a fastener or metal contact point, treat it as an unknown stain rather than assuming this table's guidance applies, and route to the chemical storage and product substitution standards before choosing a stronger mix on a guess.

References

• See related: Wood-Safe Low-Pressure Wash Standard, Wood Brightening and Neutralizing Procedure, Pressure Setting Reference for Wood Surfaces, Deck and Fence Cleaning Technique by Wood Type.
• Product SDS and label for the specific SH and oxalic-acid concentrates in use, for the authoritative source-strength and maximum-concentration figures this table's ratios are built from.