Gelcoat scratches and chips in a fibreglass boat are a fact of life. however, there comes a time when even the most hardened boat abuser starts to think about fixing things up a bit. Extensive spiderweb cracks, chunks of missing gel or wear through to the glass laminate are worth taking a look at.
The good news is that working with gelcoat is not terribly difficult. Even mere mortals can get a polished gel-coat repair. The basic procedure for making repairs is surface preparation, gelcoat application, sanding and finishing. If you follow this progression and are patient, you'll get good results.
Exterior Surface Preparation
Once you've finished sanding, wipe the surface of the repair with Acetone, making sure to remove all surface debris. Next, mask off the area around the repair. Make sure that your masking tape overlaps the edge of the sanded area, so that when you apply gelcoat, it will only be applied to a sanded surface. Deep gouges and corner chips may require filling with topcoat mixed with colloidal silica filler. Allow to dry and sand level with 200-300 wet and dry paper using a block being careful not to sand too much of the original boats gelcoat.
For repairs we recommend topcoat, this is gelcoat with a wax solution added, the wax will harden the surface making it much easier to sand. We can supply this in pre-pigmented white or, if you are adding pigment, you must use clear topcoat. First, the topcoat must be mixed with catalyst at 2% by weight. The manufacturer specifies this ratio, but in practice, polyester topcoats are fairly tolerant of imprecise mixing, Additions between 2% and 4% will cure the topcoat. Try to avoid fast mixes on hot days, when gel or topcoat cures too quickly it traps tiny air bubbles which you'll uncover when you are sanding or buffing.
Once you've mixed your topcoat, you have a certain amount of time that you can work with it. Cure time will vary with the temperature of your workspace and the amount of catalyst. This also applies to the thickness of the topcoat that you apply.
Apply a first coat thick enough to fill the gouges, keep in mind that the topcoat will shrink as it cures. Apply topcoat so that it covers the prepared area and overlapping the masking tape. Then peel away the masking tape before it cures to reveal a clean edge. If you wait too long the topcoat will harden completely, and you'll have to chip it away from the tape before you can sand. If you need to put a second coat on, you must sand the previous coat or it wont stick.