Three-stage pearl white that matched under fluorescent light
A 2019 sedan has had the right front door skin replaced after a side impact. The colour is a three-stage pearl white: white base, pearl midcoat, clear. The shop's spectrophotometer returns three variants, and the painter chooses the one closest to the fuel filler flap because the door is new and the guard and rear door are original. The booth is at 33°C and 68% relative humidity. He mixes 300 mL of base at 1:1 by volume and adds 10% thinner 'because it's hot', sprays two spray-out cards, and compares them to the guard under the booth's fluorescent tubes; the match looks close. He then sprays the door, blends the pearl into both adjacent panels and clears. In daylight at handover the door looks lighter and slightly pink next to the guard, and the blend area on the rear door shows a mottled, streaky pearl.
1. Explain the most likely reasons the door reads lighter and pinker in daylight and the pearl blend is mottled.
Read the worked answer
Several independent problems, and the light source hid the worst of them.
- Variant choice. The fuel filler flap is a small, near-vertical panel that ages differently from a horizontal guard, and it is a starting point rather than proof. Three variants exist because the plant ran more than one pearl batch; the right check is a spray-out against the actual adjacent panel.
- Thinner addition. Extra thinner beyond the maker's ratio lowers viscosity and changes how the pearl lays down. Flake and pearl orientation depend on wetness, gun distance and overlap, which is exactly how a streaky blend is produced.
- Metamerism. Fluorescent tubes have a spiky spectrum. Pearl whites are notorious for matching under one light and failing in daylight; the pink/grey shift points to pearl midcoat film build or the white base being off, not to the clear.
- Pot life. If the mixed base sat past its pot life in a hot booth, the binder has begun to crosslink, so sprayability and orientation change and adhesion may suffer.
- Heat. A hot panel plus fast solvent dries the film before the pearl can orient, giving a lighter face tone.
The door being new is irrelevant to colour; the adjacent original panels are the reference.
2. How should the spray-out and colour evaluation have been done before the door was sprayed?
Read the worked answer
- Spray the cards over the same primer and sealer colour as the job, because basecoat is semi-transparent and the undercoat changes the final tone.
- Make at least two cards: the chosen variant and the nearest alternative, plus a let-down panel showing the blend. A single card only tells you whether that variant is wrong, not which one is right.
- Apply the pearl midcoat with a consistent gun distance, pressure and overlap, and do not over-reduce it. The midcoat is where a three-stage match is won or lost.
- Let the cards dry naturally or force-dry exactly as the job will be dried; comparing a wet card with a cured panel is meaningless.
- Compare in daylight or a D65 light booth, under the booth tubes, and at both face and side angles (roughly 45° and 90°), against the guard and the rear door, not the fuel flap alone.
- Check the film build with a gauge on a test panel and confirm the clear does not shift the pearl.
Only after the card passes in two light sources should the door be sprayed. The cost of two cards is minutes; the cost of a mismatched three-stage door is a repaint of three panels.
3. The painter mixed 300 mL at 1:1 and then added 10% thinner. Show what that does to the ratio and explain the pot-life problem at 33°C.
Read the worked answer
At 1:1, 300 mL total is 150 mL base plus 150 mL reducer/hardener as the product specifies.
Adding '10% thinner' is ambiguous, which is itself a problem:
- 10% of the total = 30 mL extra, giving 150 : 180, a ratio of 1 : 1.2.
- 10% of the base = 15 mL extra, giving 150 : 165, a ratio of 1 : 1.1.
Either way the mix is off the manufacturer's ratio. That matters because the reducer/hardener controls viscosity, drying and cure; extra solvent means a thinner film, more shrinkage, poorer pearl control and possible solvent entrapment, which shows later as die-back or dull patches. It also voids any product warranty argument.
Pot life is the second issue. Pot life is quoted at 20–25°C; at 33°C the reaction runs faster and usable time can drop substantially. A batch mixed for a whole door and both blends may be gelling before the last panel. The correct response is to mix smaller batches, keep them shaded, and discard anything that has started to thicken rather than adding more solvent to keep it workable.