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
- Timing cue: rainbowing often signals the levelling window is opening or closing.
- Finish quality: unmanaged rainbowing and smear can cure into dull patches or high spots.
- Process control: helps you adjust section size, applicator load, towel choice and wipe sequence.
- Environment: heat, humidity and airflow change flash behaviour and the likelihood of smear.
Where you’ll see it
During coating application on paint, glass or PPF top coats, especially under raking light as you judge when to level and refine.
Context
Car Paint Protection; Ceramic coatings
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.
Written by Danny Argent. Last updated 07/11/2025 14:29