[flyingfrizzle]

If you are using poly-ester gel then you are better off just fairing it out with a poly type of fairing compound first. It will take care of the pin holes and also the ruff spots or low spots. Spraying a thick coat of high build poly base primer like dura tech will be better prior than just spraying straight gel on the bottom. Block the high build primer down between coats and get it close as possible then spray the gel as even a possible on top of that. Once the gel sprayed it will have to be blocked down and buffed out to a shine. If you don't get it true before you apply it you will sand threw the gel on high spots and have to re-coat it. Rolling it will make for a lot more sanding. You also don't want to fill the low spots with gel due to if the gel gets to thick it will crack. To thin is also a problem. Gel needs to maintain a cetin thickness to hold up. Dura tech also has gel additives that will help against water intrusion. That is the problem with the poly gel, it is porous and will over time suck water in. A good epoxy barrier coat will prevent this but then you can Not coat poly over epoxy or it will not stick well. You can do epoxy over poly but never poly over epoxy. They make vinyl-ester gel coat that will provide you better water protection than the standard poly. This will prevent blister in the long haul at a little more cost. You can even install the ve gel over an epoxy barrier coating for better protection. I usually only gel coat molded parts due to the high labor evolved in buffing out the sprayed / rolled on gel coats. You will sand either way but no buffing if you use a 2 part urethane over epoxy.

This is what I usually do:
I use 2000e interlux epoxy primer most of the time as a base primer. It is a high build, high solids 2 part epoxy that goes on thick. I spray several coats until I get the required thickness which is about the same as the thickness of a dime. I use epoxy water tight 2 part filler (fair compound) on the low spots of scratched and gouges over the first coat of 2000e to take care of the big stuff. Once the bad spots are took care of on top of the first coat of primer then I do several more coats and block it down between those coats to get it even straighter and truer. I try to buy two different colors of the high build primer (white & grey) so that you can alternate them between coats so that once sanded the high spots show better plus you can see you coverage better on recoats as you spray over the last. Once it is close I use interlux epoxy 2 part primekote or awlgrip 545 primer over that as a final primer seal coat. The 2000e is a barrier coat and seals up the old glass and prevents water intrusion. The primekote or 545 is a final pre-paint primer that goes down prior to spraying the final product. After this I use awlgrip or interlux perfection as a top coat. Three coats with light wet sanding in between them. The final coat I don't touch. If you use awlcraft instead of awlgrip then you can wet sand and buff plus touch up in the future.

Either way will yield good results, but you will get better protection with the ve / epoxy products over standard poly gel. I would at a minimum use a dura tech additive or step up to a vinyl ester gel if you are set on spraying / rolling on gel coat. Talk to Dave (blueheron) he had a good result on his 25 seafari doing the bottom of his boat in gel coat. I think he thinned and sprayed it from a paint gun with a large tip. He did not have much sanding or buffing doing this way as you would using the other methods above. You got to have everything super fair tho going this route just like using paint cause you wont have the extra thickness to sand down into afterwards as you would with spraying with the cup gun or rolling the gel. Maybe he can give you some tips if you want to try to thin the gel and spray in like paint.

What ever you do just be prepared for many hours of sanding and buffing to get the gel to look good as it dose out of a factor mold. Main thing- get it close before you apply anything. The prep work is what makes a final coat look good. There are many ways to skin a cat, and positives & negatives of both. Also make sure you under coats are compatible with the top coats, don't mix polyester over Epoxy. Some ways are more forgiving than others and can be repaired better than others. That's one good thing about gel coat, it is repairable better than some paint top coats but you don't get the water protection of the epoxy products. Like said before there are more than one way to do it, just make sure you prep / fair it well and apply it correctly whatever way that you pick to do it. I would do what you are most comfortable doing.