When restoring older cars, you will probably need to consider repainting different parts. While metal paintwork is often a significant part of a restoration project, you will also often need to paint plastic parts, especially on the doors and bumpers. If you need to paint plastic parts, make sure you get the result you want and avoid the following common restoration mistakes.

Treating all plastic parts the same

Auto manufacturers commonly use two types of plastic for car parts. Thermoset plastics are hard, tough and scratch resistant. After manufacture, you cannot reshape these plastics. Conversely, thermoplastics will change shape once heated, although they will lose strength when this happens.

It's not always easy to tell these parts apart. Parts that get hot (such as engine parts) are more likely to be thermoset plastics. Exterior parts are often thermoplastic, but you can sometimes tell the difference with the ISO number on the reverse of the part. Failing this, it's a good idea to talk to your auto repair shop for more advice.

Understanding these properties is important when it comes to the preparation technique you decide to use because you need to understand how the plastic part may react under certain conditions. For example, excessive sanding can damage thermoplastic parts, especially if you use a mechanical sander, because the heat of the machine can melt the plastic. With older thermoplastic parts, you should only sand by hand, so you don't make the plastic too warm.

Failure to remove imperfections

While you don't need to worry about rust on plastic car parts, you must still deal with any surface imperfections that can ruin the finished effect. Many plastic car parts develop different surface imperfections over time, especially on parts of the car like bumpers or door handles. Use a medium coarse sandpaper to carefully smooth out these imperfections before you apply primer or paint, or you may end up with a messy, uneven finish.

Using the wrong cleaning products

Before you can apply a primer or a paint, you need to make sure the plastic part is as clean as possible. Washing with detergent and water is a crucial first step, but you will also need to use something to get rid of wax and grease.

If you use a solvent-based wax and grease remover, you may damage the plastic. Raw plastic absorbs solvent products. As such, an alcohol-based cleaner is likely to give you better results. 

Failure to use adhesion promoters

Plastic adhesion promoters are specially designed chemicals that improve the bond between a primer and the plastic's surface. These special solvents open the pores of the plastic to allow the primer to stick properly to the part's surface.

Make sure you follow the manufacturer's instructions. For example, you must normally apply a specific number of coats to get the right result. What's more, you normally have a finite window in which you can then apply the primer. Leave it too long, and the primer will still not adhere properly to the plastic's surface.

Failure to use anti-static agents

Plastic parts have another property that can make life difficult for you. Plastic can take on static charge, which may cause problems with any primer or paint that you then apply.

Anti-static agents use a conducting polymer and a solvent that contains water and alcohol. After you apply the agent to the plastic, the solvent evaporates, leaving behind an invisible skin on the plastic that stops static building up. Apply the anti-static agent after you clean and sand the plastic, but before you seal or paint the part.

Drying the plastic rapidly

It's sometimes tempting to force dry painted plastic parts, especially if you quickly want to put them back on the car. Unfortunately, if you speed up the drying process too much, you may cause adhesion problems for the paint.

You should always follow the manufacturer's instructions and leave enough time for the suggested drying period. If the instructions say that you should leave the paint to dry for five hours, make sure you have five hours to wait before you do anything with the part!

If you plan to restore an older car, it's quite likely that you will need to paint plastic parts. Take time to learn how to repaint plastic properly, and choose the right products, or you may find yourself repeating the job time after time.

For more information and options, talk with auto paint professionals, such as those at Space Age Auto Paint Store.