Nearly every paint problem on a polycarbonate shell has one of four causes: the surface was not clean, the paint was wrong for the plastic, the coats were too heavy, or the conditions were cold or damp. Find the symptom below, then the fix.
Find the fault
| What you see | Likely cause | What to do |
|---|---|---|
| Paint flakes off in sheets | Paint not made for polycarbonate, or the shell was not washed. | Strip and repaint with polycarbonate paint. See below. |
| Small round craters (fish eyes) | Grease, silicone or finger marks on the inside. | Let it dry, then mist more light coats over the top. Wash better next time. |
| Runs and pools in the corners | Coats too heavy, can too close. | Leave them. From the outside they are usually invisible. Let them dry fully before the next coat. |
| Colour bled under the mask | Mask edge not pressed down. | Pick it back while fresh, or cover it with a pinstripe or decal on the outside. |
| Colour looks thin or patchy in sunlight | Not backed. | Add white or silver behind it, then black. This works even weeks later. |
| Light colour looks dirty | A darker colour was sprayed behind it. | Cannot be reversed. Cover with a decal, or strip and repaint. |
| Cloudy or frosted plastic | Very heavy wet coats, or a harsh solvent. | Permanent in that area. Hide it with a decal, or use it as a matt panel. |
| Dull, rough finish that looks dusty | Can held too far away, or sprayed in the cold. | A further, slightly wetter coat usually smooths it. |
| Overspray on the outside | Protective film removed too early. | Polish it off gently with a plastic polish and a soft cloth. |
Remember which side people see
Runs, dust and uneven backing on the inside rarely show from the outside. Before you decide a shell is ruined, turn it over and look at it the way everyone else will.
Fixing bleed under a mask
- While the paint is fresh
Lift the bleed with a cocktail stick or a cotton bud. Work from the clean side towards the paint so you do not drag colour across the window.
- Once it is dry
Scrape gently with a wooden stick or a fingernail. Do not use a blade flat on the shell; it scratches and the scratch will show.
- If it will not shift
Put a window surround decal or a strip of fine line tape on the outside, over the edge. Most professional-looking shells have exactly this.
Stripping a shell
Sometimes starting again is the right call. Be careful what you use.
Acetone, cellulose thinners, brake cleaner and most paint strippers attack the plastic itself. The shell turns cloudy, then brittle, and cracks on the first impact. Do not use them.
- Use a paint remover sold for polycarbonate RC bodies and follow its instructions.
- Test on an offcut or a hidden corner first and leave it a few minutes to see how the plastic reacts.
- Work in small areas, wipe away, then wash the whole shell in warm soapy water before repainting.
- Paint that has cured for months is much harder to remove than paint from last week.
Repairing a cracked shell
A crack keeps growing until something stops it.
- Stop the crack
Ream or drill a small round hole, 2 to 3mm, at the very end of the crack. A round hole spreads the stress and the crack stops there.
- Clean the inside
Wipe away dust and grease around the damage so tape and adhesive can grip.
- Bridge it with tape
Stick body reinforcement tape across the crack on the inside, extending a couple of centimetres either side.
- Add a flexible adhesive over the tape
A thin layer of flexible, rubbery adhesive worked into the mesh makes a patch that bends with the shell. Leave it a full day to cure.
Torn body post holes
A hole that has pulled into a slot can be rescued the same way: reinforcement tape and adhesive on the inside, then re-ream the hole through the patch once it has cured. A washer or body mount pad under the shell spreads the load in future.
When to retire a shell
When the nose has more tape than plastic, it has earned its retirement. Keep the old shell for practice days and for testing new colour combinations, and start again with a clear shell. The first guide is How to paint an RC car body.