- Home
- Knowledge Base
- Glossary
- Rainbowing / Smear
Rainbowing / Smear
Quick answer: Rainbowing is the iridescent, oil-slick sheen you see when a coating film is uneven or mid-flash; smear is the streaking or drag marks left by wiping too early, too thick, or with saturated towels. If left to cure, both can become visible high spots.
What it means
Rainbow colours come from thin-film interference when solvent-rich or uneven areas of a coating refract light differently. Smear appears when the film is moved before it has flashed enough, or when residue is pushed around by a loaded or fluffy towel. Both are application signals, not product failure.
Why it matters
Colour appearing in a wet coating is a clock rather than a defect. As the film flashes, interference patterns show up across the surface, and their arrival tells you the levelling window is opening; their disappearance tells you it is closing. Reading that correctly is most of what separates a level coat from one full of problems.
Miss the window and the same patterns cure into the finish as dull patches and high spots that then need polishing back out. What it is really giving you is process feedback: section size, how heavily the applicator is loaded, which towel is being used and in what order all show up in how the rainbowing behaves. Conditions move the target constantly, since heat, humidity and airflow all change flash times, so the same product on the same car behaves differently in July than it does in November.
When rainbowing shows up
Coating application on paint, glass and PPF top coats is where it appears, and it is watched under raking light while judging when to level and refine. Installers describe a panel as starting to rainbow or as having gone past the window, and job notes record levelled at first rainbow or smear from late wipe. It comes up most in training material, because it is the cue that separates an experienced applicator from a nervous one.
Context
Rainbowing is one of the first things our workshop watches for in the levelling window; catching it early and adjusting section size is far easier than trying to rescue a panel where the coating has started to set unevenly.
Common mistakes
- Chasing rainbowing with heavy pressure instead of timing the level-and-buff correctly.
- Re-wetting far outside the open window, which can build thickness and cause high spots.
- Using saturated, fluffy or mixed-fibre towels that smear rather than level.
- Working too large an area so overlaps cure unevenly.
- Wiping without proper lighting and fresh towel faces, missing edges and curves.