Do you know if you have Leaxn rudders or fiberglass? An easy way to tell is grab the rudder at the tip and try to bend it. Lexan bends fairly easily, glass won't. Rattle can scratches very easily, and I'd hold off using it if you can. If the rudders are Lexan, Oxalic acid and a scotchbrite pad followed with wetsanding to 800 will do wonders . If they are glass, they're worth fixing right with glass and gelcoat. Paint and Hobies is not the most ideal mix.

If they're old style black EPO's:
http://www.youtube.com/watch?v=jNwMgX48uwQ