Run your fingernail across the scratch. If your nail catches, the scratch is too deep to polish out. That’s the fingernail test, and it’s the rule of thumb we go by when deciding whether a car scratch can be buffed out.
If your nail glides over it, the scratch is in the clear coat and polishing can usually remove it.
How to do the fingernail test
- Clean the spot. Wash and dry it so dirt in the groove doesn’t fool you.
- Go across, not along. Drag your fingernail across the scratch at a right angle, with light pressure.
- Feel for a catch. If your nail slides over smoothly, it’s a clear coat scratch. If it catches or drops into a groove, it has gone through the clear coat or close to it.
Why a scratch that catches your nail can’t be removed
Polishing doesn’t fill a scratch. It cuts the clear coat around the scratch down until it’s level with the bottom of it.
Factory clear coat is only about 35 to 50 microns thick, and only a small part of that can safely come off. A scratch deep enough to catch your nail goes deeper than you can safely cut. Chasing it means thinning the clear around it, and the paint can fail later even if you never go through. We cover how much clear you have, and how much is safe to remove, in paint thickness gauge readings.
What the color of the scratch tells you
Look inside the scratch after the fingernail test:
| What you feel and see | How deep it is |
|---|---|
| Nail glides over, scratch looks white or hazy | Clear coat only |
| Nail catches, the car’s color is still at the bottom | Into the base coat (color) |
| Gray or white primer, or silver metal, at the bottom | Through the paint |
The fingernail test is a rule of thumb, not a measurement. It tells you roughly how deep a scratch is, not exactly.
What to do with a scratch your nail catches
- Small scratches and chips: touch-up paint matched to the car’s paint code fills the groove. It won’t be invisible, but it covers and seals it.
- Primer or bare metal showing: we would cover it with touch-up paint soon, so the primer or metal isn’t left exposed.
- Long or deep scratches: these need a body shop to repaint the panel.
Polishing can sometimes make a scratch that catches your nail less noticeable, but it won’t take it out. We wouldn’t keep cutting at it hoping it will.
Overall
Overall, the fingernail test is the quickest way to tell whether a car scratch can be buffed out. If your nail glides over it, the scratch is in the clear coat and polishing can usually remove it. If your nail catches, it’s too deep to polish out, because factory clear coat is thin and only a small part of it can safely come off. For those scratches we would go with touch-up paint matched to the car’s paint code, or a body shop for long or deep ones. It’s a rule of thumb, not a measurement, but it tells you which way to go before you start polishing.