See full version: How to Gelcoat Fiberglass
Gelcoat is a liquid that hardens to form a thick layer that's used to protect fiberglass and give it a shiny, smooth finish. Applying gelcoat to fiberglass is simple if you have the right tools and take care to mix your gelcoat properly. more
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You can get this at any West Marine Store or online. here
What you need to do is grind out the damaged area and repair the fiberglass. I would suggest you use real fiberglass like the West System. You need to repair it with fiberglass mat and resin. here
West Systems has a great book on How-to. [links]
If you have damage that is down to the fiberglass, I would NOT recommend using marinetex. Marinetex is used for emergency fixes and not meant for long term repair. It is like bondo for cars. here
If you are going hide the entire repair under a Keel guard, you do not need to gelcoat over the patch. However, I would gelcoat over the patch so that it looks like the rest of the boat. You can get the right gelcoat from Gelcote in Canada. They supply the gelcoat for all Seadoos. They have a page where you pick your boat and they tell you what gelcoat to get. On a white area, the match is perfect since white doesn't fade as much as colors do. You don't need much since you can spray it in using a Preval spray can which you can get for about $5.
* Gelcoat is quite viscous and almost impossible to spray without thinning it. The less you can thin it the better the gelcoat will be, so the use of a larger spray tip is required. I use both 2mm & 2.2 mm HVLP gravity feed cup guns for most of my heavy shoots. At this size the thinning is kept to a minimum.
An alternative to using surfacing wax in the gelcoat mix is to over spray the gelcoat with PVA, (polyvinyl alchol). The problem with this method is getting the right amount of material misted onto the un-cured gelcoat to block the air out and in gauging just when to apply the PVA. Too much will adversely affect the gelcoat finish & cure, applied too soon will do the same. Not enough will allow air, leaving a tacky cure, too late will not block the air soon enough still leaving a tacky gelcoat cure.
On the third coat add about 10% surfacing wax so that after the gelcoat cures it will be sandable. Allow at least 24 hours for final cure & before sanding. Sanding with 180 grit on the 3 coat will allow you to remove most, if not all of the orange peel texture and check your fairing.
Do not thin your gelcoat with acetone, MEK or lacquer thinner. Thinning with these materials can lead to an un-curable application that will aligator on the next shoot, destroying all the time and materials for both shoots, (been there, done that, got several T-shirts to prove it. ). The best product to use, (manufactured by several resin companies), is called "patch booster". Patch booster is a high quality, low viscosity polyester resin with additives to minimize a tacky surface when cured. Depending on the tip size of your gun, 10% - 15% patch booster is all you may need. Try not to exceed 20% in the first three coats.
Once you've reached the end of your sanding, the 1,000 grit will have brought your surface up to a dull luster. A course rubbing compound shouldn't be necessary and you may be able move to a fine compound like 3M Finesse-it. Follow the Finesse-it with an automotive finish sealant such as Meguire's Machine Glaze. This is followed with a swirl remover such as 3M Perfect-it & a foam pad.
Watch Crownline Video on Why Our Gelcoat has the highest luster rating in the industry
IMPORTANT: Foot traffic on soiled fiberglass surfaces will severely scratch and mar the finish. Keep the fiberglass as clean as possible.
IMPORTANT: Air temperature must be above 65°F (18°C) to apply gelcoat. The area must be clean, dry and free from oil or wax before attempting to repair.
Proper care and maintenance of the gelcoat will keep your boats luster and underlying fiberglass in good condition. Read these tips for maintenance and repair advice on everything from cleaning to repairing cracks.
The deck and hull are made of fiberglass covered with gelcoat. Gelcoat is a scratch resistant, color-pigmented, polyester resin. With normal use, some damage to the gelcoat should be expected.
Minor scratches can be repaired with car wax. Treatment will repair or lessen scratches. Apply wax to a damp, soft cloth. Rub the gelcoat surface gently with the cloth in a circular motion. here