Macro,
Some threads below which deal with both polished or sanded bottoms and fins. 120grit is way too rough - depending on severity I usually start with 400grit then 800 and finish with 1200/1500 grit and sand in the direction of flow (with a sanding block too as with the rougher grit you can get flat spots). If the fin has bad nicks in leading edge it's best to fill these with epoxy (tape one side of fin with about 5mm of tap past the edge and then dab drops of epoxy on nicks (the tape holds the epoxy in place) otherwise if the fin is lightly scratched you can apply spray putty and then sand back. Spray putty is excellent although doesn't make you fins look pretty unless you overspray with paint. If the leading edge is severly damaged you can apply a strip of 4oz glass around the front edge and tape down really tight. Make sure you squeeze any excess resin out as it's a PITA sanding small ridges of resin out. You can sand most of the glass off and be left with a beautiful new leading edge.

Don't make your tip too sharp either as the damage it can do is not worth the extra .02knots you're going to get if it's razor sharp.

www.seabreeze.com.au/forums/Windsurfing/Gps/Ultimate-High-wind-rough-water-Speeeeed-tips/

www.seabreeze.com.au/forums/Windsurfing/Gps/To-sand-or-to-polish-that-is-the-question/

www.seabreeze.com.au/forums/Windsurfing/Gps/polishing-bottom-of-boards/

www.seabreeze.com.au/forums/Windsurfing/General/fin-surface/

www.seabreeze.com.au/forums/Windsurfing/Gps/Speed-board-finishes-which-is-faster/