Nice sanding job! You are off to a great start.
I'd go with Gene's recommendation; definitely start off with washing the entire rudder down with acetone. However I would *not* thin epoxy - *ever*. I would either buy penetrating epoxy, or (my preference) get a slow-cure epoxy, gently warm it to reduce viscosity, and then paint it on. You will find that cool (winter) epoxy has the consistence of molasses, warm (midsummer) epoxy almost like water!
Thinning epoxy (e.g. with acetone) can significantly effect the properties of the material. I would not recommend it.
Other thoughts: You may find a 'modified twill' cloth will help wrap around the curves. Have a look at TAP Plastics
. Don't lay up more than 3-4 layers at a time or it could (will) overheat and damage the layup. Because of the curves, you may want to consider alternating the wraps around the rudder (wrap one side overlapping ~ 4 inches; then turn over the whole rudder and wrap the other side....). You way want to tape a layer of plastic on to your work surface which the epoxy will not adhere to - your fiberglass supplier will carry this. The plastic sheeting is also useful to lay against the layup to give a smooth surface, and reduce the need for sanding. Finally - if you have not done this before - remember the less resin, the better. The cloth needs to be evenly wetted out, but not "floating" in epoxy. Use a plastic scraper to remove excess resin.
Personally - I know that this is a bit funky! - in high-abrasion or tension areas I have been known to sandwich a layer of Kevlar between the 'glass layers (I say between as you can't sand kevlar!) For example, I once repaired a transom which had been repaired at some time in the past, but had cracked open again. With a kevlar sandwich, that crack never reopened!
Finally - most importantly - enjoy! Fiberglass work is fun and a lot easier than it seems, Good luck!