You were probably warned by Foss Foam that you should not use any dark colored paint
- antifouling, etc. on the rudder as the foam core
can bake and shrink.
A barrier coat might not be a bad idea to keep the rudder from absorbing water
since the surface is only a gelcoat
over the FRG skin. There are quite a few brands of barrier coat and for such a small job then I might suggest the Pettit Protect available in quarts.
If the boat is going to be left in the water with the rudder attached then I might suggest the new Pettit Econea paint which comes in white and is available in quarts.