This Verses has been going on for a long time and I have seen many mis-informed opinions in many Internet forums. This article is intended to provide that facts and then let you decide which one you will choose. So lets start with a little background about Gelcoat.
Gelcoat is a cosmetic layer of Polyester Resin, thickening agents and colour pigments. It's sole purpose is to provide a quick, smooth, high gloss finish to production molded boats and marine watercraft. It has a consistency is like gravy and has NO self leveling qualities. Gelcoat is used in the manufacture of production Fiberglass boats that are fabricated using a female mold. It is applied using special spray equipment. The Gelcoat layer is the first to be applied to the inside of the mold once the Mold surface has been treated with a release agent. After this Gelcoat layer has cured to a tack, layers of fiberglass fabric embedded in polyester resin are applied to this layer. This is done in several succeeding layups until the desired thickness of the structure has been achieved.
So now lets talk about what can go wrong when mixing and applying Gelcoat.
- You apply the Gelcoat too thin - Blotchy appearance when cured
- You apply the Gelcoat too thick - Risk Spider Cracks in future
- You under catalyst the mixture - Gelcoat does not dry
- You over catalyst the mixture - Gelcoat "Kicks Off" before you can apply it (30-45 minutes)
- You don't apply three 10 mil coats - see #1 & 2
Now lets assume that everything has gone well and you have successfully applied 3 coats to achieve an overall thickness of 30 mils. No matter how carefully you applied the Gelcoat by brush and roller, you will need to do a lot of sanding to achieve a uniform gloss and appearance with the original Gelcoat. For the finest finish, you'll want to wet sand through 1,000 grit before compounding. Make sure that each grit has removed the scratches from the previous grit. Don't move to the next grit without thoroughly rinsing. A wet surface won't allow you to see fine sanding marks, so dry the surface and inspect it before moving to the next grit... 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.
OK so that is the Gelcoat Process and here is the Painting process. Use a good quality marine urethane such as Interlux Brightside which has excellent application characteristics and yields that “sprayed on” look when brush applied in thin coats and is ideal for use anywhere above the true waterline. Brightside Polyurethane is easy to clean, resists staining and has great abrasion resistance. A hard, high gloss one part polyurethane finish. Please note that the "above the waterline" disclaimer refers to boats that spend most of their time in the water, trailered boats can use this product on the hull. If the boat lives in the water then an antifouling coating would be appropriate.
In closing I should point out that using gelcoat to repair minor dents and scratches is not as arduous as described about, however matching the original gelcoat colour can be a challenge due to aging of the original surface.