This brings back some memories for me....
I learned to do fins the right way at Jim Phillips' place in the early 80s. If Jim has posted on this before, it would be good reading.
Or you can check out Greg Tate's ''how not to build fins'' thread, one of the funniest things I've ever read on here.
I don't do fins much anymore, but I still remember a thing or two....
Tips on symmetrical sheets for double foil fins:
1) Mix all your resin colors first.
2) Hotcoats are needed to get crisp color change in finished fins. Usually you'd lay up your first color, hotcoat (with same colored resin + wax), and as soon as it kicks firm, put down an ''accent line'' (a hotcoat of a contrasting color). Then go to another color layup, etc.
3) If your hotcoats don't come out flat, or you let the HC cure completely, you need to sand before doing next layer(s).
4) If you've got exotherm issues, stop and let it cool.
5) Think about what it's going to look like when foiled. Example; transparent outer layers over contrasting accent lines are going to ''fade'' when foiled, a nice look. All kinds of things are possible. It's always fun to rip into the first fin out of a new sheet and see the colors reveal themselves...
1) When you cut your fins out of the sheet, don't cut the base. Mark it, but leave the excess hanging. It gives you a tab to clamp on, and you can foil past the baseline. Cut the base off last. Cut a slight curve in base, not a straight line. Then it won't ''rock'' when you tack it on prior to glass-on.
2) Cut ''bands'' when foiling. The big one that takes the meat out of the back of the fin is the hardest and most critical. Use a hard disc, not a powerpad, for the rough-out.
3) Don't scrub on spots. Smooth ''passes'' with grinder are best.
4) Don't sweat small wiggles in outline, they'll be much easier to clean up near end of process when the outline is thinned considerably.
Best tip of all: HAVE FUN!!!