A crack in plastic may be properly repaired by welding and repainting. However, this type of repair is expensive and requires a professional. If the crack in your plastic does not require extensive repair, you can easily conceal a crack in plastic using simple household and store-bought items.
Clear the damaged surface of the plastic, removing decorative or protective pieces that may prevent access to the crack. Trim any pieces of plastic protruding from the crack. Sand the damaged area with medium-grit sandpaper.
Clean the surface of the plastic with a degreaser to remove oil residue, plastic fragments and dust. This will provide a clean surface to which the plastic filler or putty can properly adhere.
Prepare the plastic epoxy putty, carefully following the manufacturer's directions. Be mindful of the curing time for the epoxy putty — some epoxy putties have short curing times, so try to complete your work within that timeframe, working quickly but methodically.
Push the putty into the crack and spread the putty over the damaged area using a flat mixing paddle or any sort of implement that can be used as a trowel. Smooth out the putty using the paddle.
Trim or remove the excess putty. Allow the putty to cure and harden.
Lightly sand the patched surface using fine-grit sandpaper. Remove all traces of dust. If necessary, apply a second coat of putty in the same manner as in the previous steps.
Apply one coat of all-purpose alkyd primer using a paintbrush. Allow the primer to dry.
Apply acrylic paint, adding extra coats of paint if necessary. Allow each coat to dry before applying the next coat.
Gently sand the painted surface with fine-grit sandpaper. Remove all traces of dust.
Apply a light coat of auto wax or furniture polish to the patched area using a clean cloth.
To stop the crack from spreading, drill a small hole at the end(s) of the crack.
If the crack is severe or wide, you may need some sort of bracing, such as tape, on the underside of the damaged area, if accessible, so the putty may have a surface to which it can adhere.
You may also use a plastic filler in place of the epoxy putty.
Choose a paint color that matches the color of the plastic. If no paint color matches the color of the plastic, mix two or more paint colors to get a color that matches the color of the plastic.
Work in a well-ventilated area, as some plastic fillers and epoxies may emit noxious vapors.