How To Prep Fiberglass For Paint: Basic Fiberglass Preparation Process Before Painting

Basic Fiberglass Preparation Process Before Painting

Fiberglass, also known as GRP, for glass-reinforced plastic or FRP for fiberglass reinforced plastic, is regarded as a kind of plastic that is soaked or saturated with small glass fibers for reinforcement. Fiberglass is lightweight, strong when it comes to both compression as well as tension, and easy to mold into intricate shapes. Fiberglass was first introduced in the aircraft industry and has gained wide acceptance as a material for boat hulls, car bodies, and even residential construction since then. If you are looking to paint your fiberglass, you will, first of all, need to prepare your fiberglass for painting.

Fitting And Trimming Fiberglass

  • After fiberglass gets manufactured some of its parts can take weeks to cure. The minimum curing time is generally 48 hours, however, some fiberglass parts should cure for weeks. If you are not going to install the part immediately, then it is important to clamp the piece to a frame or other rigid structure in order to keep away from any warping or twisting. Make sure not to hang fiberglass parts from the rafters or pile them in the attic. Gravity, excessive heat, as well as time, can be some of the worrisome factors when it comes to final installation.
  • After you are done with the curing process, you need to inspect the body panels, fenders, doors, or any other components for proper fit. Aftermarket fiberglass bodies were never put together at the factory, therefore, you can not just assume the panels will fit perfectly. Sometimes you will need a little help and if you didn’t put together the parts before curing, make sure to do it before sanding and priming. Don’t simply hang the doors before assuming everything else will fit, some parts may just fit better than others.
  • Trimming the panels is an easy task that can be done with the help of a grinding disc or cutoff wheel. It is important to protect yourself, due to the fact that fiberglass is not something that you want in or on your body. Wear eye protection as well as a mask so that you don’t inhale the glass dust, and protect your skin with long pants, sleeves, as well as gloves. You can use shims in order to properly gap the fenders or fit the doors. Remember that with the flexibility of fiberglass, shimming in one place may get rid of the gap somewhere else.


  • As soon as you are fully satisfied with the fit, clean the body rigorously in order to get rid of any surface contaminants including grease or mold releases. A good wax, as well as a grease remover or alcohol with the help of clean rags or paper towels, should be used at first to clean the panels. One problem that you will encounter using a Scotchbrite pad with cleaners first is that it has a tendency to open up the pores in the gel coat that can allow some of the release agents to soak up into the fiberglass. Prevent cleaning like this due to the reason that it can cause imperfections in the paint down the road.
  • As soon as the panel is scrapped satisfactorily, run over it an extra time with wax and grease remover or alcohol and wipe it down using clean paper towels in order to get rid of any final residue. Use wet paper towels before wiping the panel down in small sections at a time. Dry thoroughly using clean paper towels and make sure never to allow the cleaner to air dry because it will leave contaminants on the panel.


  • Now, it is time to sand the fiberglass before you start painting. It is important that the body panels are well supported, otherwise, there is a risk of distorting the panel with pressure from the sanding block. One of the best things about fiberglass is the gel coat finish, which will serve as an initial guide coat. You will need to sand the part as long as the gelcoat is moderately dull to give it enough tooth to take hold of the primer. The gel coat will display any low spots in the panels by remaining glossy, while the surface on every side of it will look dull.
  • Start by dry sanding with the help of 220-240 grit production paper in order to rigorously get rid of the glossy layer from the gel coat. Any areas that are still left glossy will not give the primer a tooth to bite into for adhesion. Be extremely careful to avoid sanding completely through the gel coat 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, you need to seal it with fiberglass resin or body filler. Corvette panel adhesive is one of the best products in order to repair these areas. Repairs such as these always need to be sanded and blocked smooth before you lay down your first coat of primer. Any major repairs or alterations generally need extra procedures to achieve professional results.
  • The primer used should be consistent with the topcoats that you are using. If you are looking to use today’s modern base or clear coats, then urethane primers need to be used, which also applies to any single-stage topcoats. Remember, you can always go through the recommended procedure from your local auto body supplier. The primer needs to be wet sanded using 400 grit paper or finer to mate with the top coats being used. Be sure to rigorously dry and wipe down the surface using clean paper towels and do any extra cleaning with a final wash or wax and grease remover.
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