The Adhesive Library

Repair guide

How to reattach ABS trim to a polycarbonate sign cover

Polycarbonate is the fragile half of this joint. It crazes on contact with the solvents in most plastic adhesives, so the safe choice is a polyurethane adhesive that grips without attacking the surface.

The short answer

Use a polyurethane adhesive, and keep every solvent away from the polycarbonate. Solvent cement and many cyanoacrylates will craze polycarbonate into a web of fine cracks, which is permanent and usually shows up days later rather than straight away. Abrade the ABS trim, clean the polycarbonate with soapy water and nothing stronger, apply the polyurethane in a thin bead and hold the trim with tape until it cures.

Difficulty
Difficult
Time needed
30 minutes work, 24 hours cure
What you need
Polyurethane adhesive, plastic scraper, 180 grit abrasive paper, low tack masking tape, washing up liquid, clean lint free cloths

Sign covers fail at the trim because the two plastics expand at different rates in the sun and the joint is right at the edge where the leverage is worst. Whatever you bond with has to give a little.

Polycarbonate's weakness is stress crazing. When it is under load, which it always is at a bonded edge, contact with the wrong solvent opens microscopic cracks in the surface that spread and go cloudy. Isopropyl alcohol, acetone, and the solvent carriers in cyanoacrylate and solvent cement all do this. The crazing can appear a week after the repair looks perfect, which is why so many sign repairs fail in a way that seems unrelated to the glue.

Polyurethane adhesive avoids the problem. It cures with moisture rather than by flashing off a solvent, it stays slightly flexible so the differential movement does not concentrate at one point, and it bonds well to abraded ABS.

Step by step

  1. Remove the old adhesive mechanically with a plastic scraper. Do not use a solvent remover on the polycarbonate.
  2. Wash the polycarbonate with warm water and a drop of washing up liquid, rinse, and dry with a clean cloth. That is the only cleaning it gets.
  3. Abrade the bonding face of the ABS trim with 180 grit and wipe the dust off with a dry cloth.
  4. Dry fit the trim and mark its position with low tack tape either side.
  5. Run a thin continuous bead of polyurethane adhesive along the ABS trim, not the polycarbonate.
  6. Press the trim into place with firm hand pressure and tape it every 100mm.
  7. Wipe any squeeze out immediately with a dry cloth. Do not use solvent to clean it up.
  8. Leave the tape on for 24 hours. Polyurethane cures from the outside in and needs the time.

What to avoid

Do not use solvent cement. It will bond the ABS happily and destroy the polycarbonate at the same time.

Do not clean polycarbonate with isopropyl alcohol, acetone, methylated spirit or glass cleaner containing ammonia. Soapy water only.

Do not use a heat gun to soften the old adhesive. Polycarbonate distorts long before the adhesive lets go.

Polyurethane adhesive foams slightly as it cures and it sticks to skin stubbornly, so wear gloves and mask the visible face. Crazing caused by the wrong cleaner is not repairable, so test any product you are unsure about on a hidden corner and wait 48 hours before trusting it.

The full bonding guide

Everything about gluing ABS to Polycarbonate

Last reviewed 13 September 2026.