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How To Prepare Fiberglass For Painting

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Congratulations! You’ve found the source for the finest fiberglass auto body parts on the planet! Our entire line of fiberglass comes with a white gelcoat finish that is ready to scuff, sand, prime, and paint. The best trick to achieve a flawless paint job on any fiberglass panel begins with the prep work. By following a few basic steps, your Speedway fiberglass will fit well, be a snap to paint and last a lifetime.

Fitting and Trimming Fiberglass

A fiberglass part can take weeks to cure after it’s manufactured. If you follow what some of the experts say, the minimum curing time is usually around 48 hours, while others say fiberglass parts should cure for weeks. If you don’t install the part right away, be sure to clamp the piece to a frame or other rigid structure to avoid any warping or twisting. Do not hang fiberglass parts from the rafters or stack them in the attic. Gravity, excessive heat, and time can make the final installation a nightmare!

After the curing process is complete, you’ll want to check the body panels, fenders, doors, or any other components for proper fit. Aftermarket fiberglass bodies were never assembled at the factory, so you can’t just assume the panels will fit perfectly. Sometimes they will need a little help and if you didn’t assemble the parts before curing, do so before sanding and priming. Don’t just hang the doors and assume everything else will fit, some parts may simply fit better than others?

Trimming the panels is not difficult and can be done with a grinding disc or cutoff wheel. Be sure to protect yourself, fiberglass is not something you want in or on your body. Wear eye protection and a mask so you don’t inhale the glass dust, and cover your skin with long pants, sleeves, and gloves. You can use shims to properly gap the fenders or fit the doors. Keep in mind that with the flexibility of fiberglass, shimming in one place may throw off the gap somewhere else.


Once you’re satisfied with the fit, you’ll need to clean the body thoroughly to remove any surface contaminants like grease or mold releases. A good wax and grease remover or alcohol with clean rags or paper towels should be used initially to clean the panels. One issue with using a Scotchbrite pad with cleaners first, is that it tends to open up the pores in the gel coat and can allow some of the release agent to absorb into the fiberglass. Avoid cleaning like this since it can cause imperfections in the paint down the road.

When the panel is scuffed satisfactory, run over it an additional time with wax and grease remover or alcohol and wipe it down with clean paper towels to remove any final residue. Use wet paper towels and wipe the panel down in small sections at a time. Dry thoroughly with clean paper towels and note never to allow the cleaner to air dry as it will leave contaminants on the panel.

Sanding Fiberglass

Time to get sanding! Before you begin, be sure the body panels are well supported, otherwise you run the risk of distorting the panel with pressure from the sanding block. One of the great things about fiberglass is the gelcoat finish, which acts like an initial guide coat. You’ll want to sand the part until the gelcoat is slightly dull in order to give it enough tooth to hold the primer. The gelcoat will show off any low spots in the panels by remaining glossy, while the surface around it will look dull.

Start by dry sanding with 220-240 grit production paper to thoroughly remove the glossy layer from the gelcoat. Any areas left glossy will not give the primer a tooth to bite into for adhesion. A warning about sanding the gelcoat: use caution to prevent sanding all the way through the gelcoat layer, which can open up pinholes in the fiberglass that can later show through the paint.

If the gel coat is sanded through and the strands of the glass show, this needs to be sealed with fiberglass resin or body filler. Corvette panel adhesive is a great product to repair these areas. Repairs such as these should always be sanded and blocked smooth before laying down your first coat of primer. Any major repairs or alterations usually require additional procedures to achieve professional results.

The primer used should be compatible with the top coats that you’re using. If you decide to use today’s modern base or clear coats, urethane primers should be used, which also applies to any single-stage top coats. Remember, you can always follow the recommended procedure from your local auto body supplier. The primer should be wet sanded with 400 grit paper or finer in order to mate with the top coats being used. Be sure to thoroughly dry and wipe down the surface with clean paper towels and do any additional cleaning with a final wash or wax and grease remover.

For a look at Speedway Motors' fiberglass parts and accessories click here.

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