I just finished applying gelcoat to my boat. Results were less than wonderful.
I have some experience, with bondo, fiberglass and painting, so it did not seem like it should be to difficult to get reasonably good results. Still I did a lot of research online before choosing a product and beginning the operation. I choose 'TotalBoat White Gelcoat' as the most promising product, I found the most negative review of the product and researched what the best methods for not repeating their experience (see: In a boat repair, what would cause Gelcoat not to harden?)
I prepped the boat, sanding, patching cracks (fiberglass), filling (bondao), sanding and finishing up with a acetone wash, before starting to apply the gelcoat in a heated, vented, humidity controlled garage.
Doing the math it came out that one quart of gelcoat would be a bit more then one coat on my 14 foot sunfish hull. Experience with rolling paint and quick setup time once harder is added, drove me to choose a brush. I was worried about the amount of product the roller would absorb combined with curing quicker then I could full apply one coat. I choose to use a brush mixing and applying gelcoat in 4 ounce batches. While the product did not give directions for mixing less then quart size batches, multiple online reference indicated 7 to 12 drops per ounce at 70 degrees F gave good results.
I purchased two quarts of TotalBoat White Gelcoat, one without wax for the first coat(s) and one with wax for the top coat. I mixed and applied a first coat just to the areas where I had sanded to bare fiberglass, I used paint strokes fore to aft. Because I had purchased relatively cheap brushes, there was some issue with loosing bristles in the gelcoat and having to pluck them out (Next time buy better brushes!) About 8 hours later it had dried nicely with a light tack as unwaxed gelcoat should. I mixed and applied a second coat to the entire hull (6 small batches), using port to starboard strokes. I let it cure overnight and the next morning it was perfect.
First thing in the morning, I began mixing and applying the waxed gelcoat as the final coat, fore to aft stokes. At 7 drops to the ounce I had only used about half of the catalyst from the first quart, so I continued using the same catalyst with the second quart. Really Really need to spend more then a dollar per brush next time! You throw them away at the end, but all those loose bristles are a major pain.
4 hours after fishing the final coat about 50% of the bottom (upside) hull was shinning over as the wax helped the final coat set completely, the gelcoat on the sides did not appear to have cured at all, it was still wet to the touch. 8 hours after finishing the final coat and there was not much change. Having done my research, I knew part of the solution was waxed paper so I covered the entire hull of the boat with waxed paper.
A new box of wax paper with 75 square feet is almost enough, had to run to the store for a second box. I left the wax paper in place until the following evening (30ish hours after final coat) when I removed it, the gelcoat was mostly cured. It was difficult to apply smoothly, some areas had wrinkles, if there was air in the wrinkle it was still tacky. The sides were still kind of wet (sticks to your finger when you touch it, as opposed to a clean finger tacky feel with the first two coats). I pulled off the wax paper, mostly it removed nicely. In the places where the gelcoat was still wet it was messy. I used the same wax paper just turned it over and reapplied to the areas that were not completely cured. Another 24 hours later the gelcoat had completely set. But is is not smooth. I had anticipated some sanding because I used a brush, but this is far from just some brush marks. Applying the wax paper to such a larger area (with a liberal application of profanity) resulted in a very uneven final surface.
While the bottom is poor, the edges and sides are really bad, It looks like a first attempt to apply mud on drywall.
It is winter now, and the boat is going outside, so the car can come in, it will be spring before I begin sanding the hull and considering the application of another layer of gelcoat to make up what I am going to have to remove to make it smooth.
What can I do next time to get a better cure, and smoother final coat in this DIY project?
P.S. This is all polyester so it rules out amine blush as potential cause of issues.