There are no resin systems, or coatings for fiberglass fuel tanks that withstand ethanol fuel at this time. Unless you have a fiberglass shell over a polyethylene tank, or metal tank it really just doesn't work when ethanol is in the mix. There are a lot of coatings that work well on metal tanks, to fill seams and rust... but the solvent (ethanol) breaks through and saturates them too, But the surrounding tank doesn't get broken down like it does with a resin based tank.
First off... try not to buy this stuff at retail at the hardware store, or west marine. The markup between a 1 gallon pail at retail, and a drum quantity at a boat building price, you can almost use epoxy for everything... So, find a boat builder near by or look at RAKA, E-bond, and some of the other brands other than West. For polyester look at Ebay, and call around for 5 gallon pails... Should be well under $100 for a 5iver.
The same thing holds true for air rollers, scissors, and paint brushes. (Check bodi.com for the tools. Paint brushes are 18 cents each if you buy a 24 count box...)
PVA is a pain in the butt as a mold release, as it adds texture to the mold that needs to be sanded and buffed out. Mold release wax, works quite well. Honey Wax is what I use, but there are better products.
Duratec makes surfacing primer that are polyester based that builds up quickly, and fills lows. 15-30 mils at one go, use a mil gauge. Go from an 80 grit finish to 800 grit wet sanding paper blocking out the eggshell, then take it to 1000, use a power buffer and buff it to a shine. I like Aquabuff 2000 on a power buffer.
Bondo is not very good for feathering out, the grain size is huge which means it needs to be top coated with other puttys to fill the texture. I do like bondo for rough work, as I don't feel bad about cutting it with catalyzed polyester resin, so that you have a slurry you can paint over open grained and loose structured stuff like drywall mud. (Easy to sand... and cheap!)
Evercoat Rage, or Dynalite pull much smoother without getting into expensive stuff. You might try a finishing coat of 3m Pirannha putty, followed by Evercoat Ultrasmooth putty pulled with a razor blade into pin holes and small scratches.
Sand the plug out with as long of a sanding board as you can use, Use a spray guide coat to find your highs and lows.
If you want to use your west systems to coat the plug, you can greatly speed up the process by using their G5 5 minute epoxy, and the 105/205 system. If you mix the 105/205, and the G5 in their own containers and then mix both together, you can make a thinner consistency product that kicks off a lot faster. A paint brush works to apply, but a squeedgee works nicely to push it into the lows. West system 407 or 410 (Or scotch light microballoons by 3m if you do a lot...) works well in place of polyester filling puttys.
For curves, battens of clean straight grained wood or aluminum bar stock work nicely. Take chalk, pencil, ink... paint... whatever and rub it on the batten, then rub it on the curve. All the highs will show, all the lows will still be clean. Then use the longest battens, straight edges, and sheet rock trowels to pull the putty between them.
Sand with long flexible boards, and on small stuff a paint stirrer wrapped in sandpaper. A body shop supply house can get you the 3m lineup. The red sticky backed inline paper is good for home made boards. 30 inches is about as long as one person can easily apply even pressure to...