Undoubtedly at some point during your period of boat ownership gelcoat repairs will be necessary. After all, a boat is a big investment (well maybe not an investment) and for the most part boaters take pride in the appearance of their boat. What is worse than a freshly cleaned and waxed boat with a noticeable scratch or chip?
All is not lost when this happens. In fact gelcoat repairs are quite easy with a little bit of knowledge and some good ol 'elbow grease.
Before making the repair you should check to make sure that the damage is contained to only the gelcoat and has not damaged the fiberglass or that there is no structural damage. If the fiberglass is damaged or you suspect structural damage, have your boat inspected by a professional.
Perhaps the most difficult part of gelcoat repair is matching the gelcoat color with your hull color. This can be easily accomplished with a little bit of research. Try contacting the boat manufacturer or research on the internet. More often than not, you will find the exact color can be ordered to match your boat. If you can not find an exact color match you will have to mix a coloring agent with the gelcoat to obtain the desired color. Mix a small sample of the gelcoat and the coloring agent, let it dry to ensure it is the correct color when it dries.
If you have a scratch or chip it is best to use a gelcoat paste. Gelcoat paste will allow you to fill the scratch or chip with minimal coating. In most cases, one application will suffice. Liquid gelcoat will take many applications to build up the scratch in order to make it flush with the adjacent surface area.
I found the following steps will produce the best finish and minimize the cleanup.
1. Preparation
When you are satisfied that the gelcoat you have matches the existing color it is time to prep the scratched or chipped area. To do this get some masking tape and mask off the four sides around the chip leaving a small area of good finish exposed (approximately 1/4 "-3/8" around the scratch on all sides). This will prevent excess gelcoat from adhering to the surrounding surface areas that have not been damaged.
When the taping is finished, sand the scratch and the exposed area around the scratch inside the tape with some 80-100 grit sand paper. You want to sand the area enough to lightly scuff up the exposed surface, but not so much that you make an indent or damage the fiberglass. Next sand the scratched area to remove any loose flakes or debris. You want to sand enough to make sure that there will not be any possible voids when applying the paste (create a "V" groove of the scratch). Gelcoat paste can bridge an opening when applied which will create a hollow void when it dries. Once you have finished sanding, wipe the area you sanded with some acetone to remove any wax or foreign debris.
2. Mixing and Applying the Gelcoat
Mix the paste and the hardener as indicated on the instructions supplied with you gelcoat. Once it is mixed apply the paste to the scratch. Do not put on to much paste or it will ooze out beyond the taped area. After the paste is applied, using a utility knife razor blade or a plastic scraper remove the excess paste with one smooth steady drag across the surface. You want to straddle the edges of the tape on either side of the scratch with the blade or scraper. This will give a finished surface of gelcoat slightly higher than the rest of the area when the tape is removed. Make sure that you level the paste and scrape off the extra before the gelcoat hardens. Do not keep trying to scrape and level the paste to get a perfect finish. Believe it or not this will only make it more difficult to sand and finish later.
3. Sanding and Finishing
After the gelcoat has fully hardened (follow manufactures drying time), remove the tape and begin sanding the repaired area with 150 grit sand paper. When sanding, do not sand the repaired area below the adjacent surface. Use a small sanding block to help ensure an even surface with the adjacent surface.
When the new gelcoat is flush with the adjacent surface, change the paper to 220 grit wet paper. Again using a block, sand the repair, this time use a small circular motion keeping the sand paper and the area slightly wet. Feather the repaired area into the old gelcoat until you can not detect a ridge or bump in the repaired area (use caution to sand the area flat if it is on a curved area).
Change the sand paper to 420 grit wet paper. Wet sand the area until the repair area has a uniform appearance and blends in with the surrounding surface completely.
Again change the sand paper using 660 grit wet sand the area and finish off with 1000 grit wet sanding paper.
Once you are satisfied with the blending of the sanded area, dry off the area. Apply rubbing compound to the repaired area. Buff the area in a circular pattern in order to get a high gloss. This may take a couple of applications, reducing pressure on every application, to reach the desired finish which should match the glossy sheen of the original surface.
Finish off with a fresh coat of wax. If done correctly, the damage should be undetectable when you finish.