Blushing, Fisheyes and Peeling on Catalysed Lacquer over Iroko

1. Milky patches - blushing. Moisture is trapped in the film. At 12 C and 78% RH the dew point is about 8 C, so the margin between air and substrate is only about 4 C and the wood itself may be colder. Evaporating solvent chills the wet film below dew point, water condenses into it, and the two-pack chemistry is also below its minimum cure temperature (typically 15 C, with RH normally capped near 75%). The water and slow solvent release leave the film hazy. A heavy 150 micrometre wet sealer coat makes it worse: more solvent to lose, a thicker skin forming first, and the moisture locked underneath.

2. Craters and fisheyes. Surface-energy problem, not a spray-gun problem. Iroko is an oily timber and its extractives, plus any compressor oil or silicone contamination from the rag, polishes or airborne release agent, create low-energy spots the wet film cannot wet. The film pulls back from those spots into craters. The concentration near knots and along one edge points to extractives and handling contact rather than a uniformly dirty panel.

3. Peeling with a powdery residue. Classic intercoat and substrate adhesion failure through an uncured layer. The sealer was sanded while still tacky, which smears it rather than cutting it, and the topcoats went on 45 minutes apart in the cold. The undercoat never cross-linked, so the topcoat is stuck to a weak, friable layer. The powdery residue is that uncured layer. Sanding to P180 over an oily timber compounds it - the surface is too polished for the sealer to key, and the dust was not removed with a solvent wipe.

Three independent causes, one common factor: the process was run outside the conditions the product needs.

Test before deciding. Do not strip 18 units on the strength of looking at them. On each unit, run a cross-hatch or knife-lift adhesion test in three places and a solvent rub with MEK or lacquer thinner on the suspect areas. A film that lifts at the cross-hatch or softens and dissolves under a rub is not cured and cannot be repaired from above. On the two units that are visibly flaking, the test is a formality; the question is really whether the other 16 are sound or merely not yet failed.

The judgement. Blush and craters are surface defects and could in principle be flatted back and recoated. But every unit was sprayed at 12 C with an uncured sealer, so the failure is in the film body, not the surface. A patch repair on a cold-cured film that is still soft will fail again in the client's building, and you will be paying twice. For units going into a heated interior with a ten-day deadline, strip all 18 back to bare timber. It is the cheaper route once you count the risk of a second failure.

Stripping procedure. Use a methylene-chloride-free stripper compatible with two-pack PU, or sand back with a random-orbit through P80, P120, P150 - not P180 straight away, since the coarser grades are what remove the film. Do not burnish the iroko surface.

Re-finishing sequence.

1. Wipe down with acetone or a dedicated wax-and-oil remover; two rag changes, and change rags often. Rags are a fire risk - bag and remove them.
2. Seal within 30 minutes of de-oiling; once the surface oxidises again the window closes.
3. Use an isolator or barrier sealer designed for extractive timbers where adhesion is critical.
4. Apply 25-35 micrometre dry film per coat, sanding between coats with P240-P320 once genuinely cured, and build 90-120 micrometre total dry film.

Programme. Stripping and re-finishing 18 window seats is realistic in 6-7 working days with one sprayer, provided the bay is brought up to temperature first. Ten days is achievable - but only if the decision to strip is taken today, not after another week of spot repairs.

Environment. Air and substrate at least 15 C, ideally 18-22 C. RH between 45% and 70%, with a hard stop above 75%. Substrate temperature at least 3 C above dew point - post a dew-point chart and a thermometer/hygrometer, and make the sprayer read them before mixing. Air movement for solvent removal, but no draught across the wet film. Filtration on the extraction so over-spray is not recirculated into the bay.

Compressed air. Refrigerated dryer plus coalescing filter, drained daily, with a final filter at the gun. Water and oil in the line cause both blushing and fisheyes and are trivially cheap to eliminate.

Substrate preparation. Sand to P150 for sealer - P180 only as a denibbing grade between coats. De-oil iroko with acetone or naphtha, two clean rags, and seal within 30 minutes. Remove dust by tack cloth and by vacuum, never with a dry rag that redistributes it.

Product handling. Acid-catalysed lacquer needs an induction time of about 15-30 minutes after adding catalyst before spraying. Observe the stated pot life strictly - at 20-25 C it is typically 8 hours, less in a hot bay - and mix only what will be used in that window. Record the batch and the catalyst ratio; a mis-weighed catalyst gives a film that never cures.

Film build. Sealer and two topcoats, 25-35 micrometre dry per coat, 90-120 micrometre total. Measure with a comb gauge on a test panel or a spare offcut from each batch, not by eye. Over-thick coats trap solvent and cause blushing and solvent popping.

Between coats. Denib when the film is hard enough to powder rather than drag, then dust off. Minimum recoat interval per the datasheet, not 'when it feels dry'.

Controls that make it stick. One spray test panel per batch, kept and labelled. A daily bay log: temperature, RH, dew point, product batch, catalyst ratio, film thickness. And a stop-work rule the sprayer is allowed to use - if the bay is cold or damp, the job waits. This job cost more in rework than the heating would have cost for a month.