Actually, I must say, the appliance epoxy works OK on EPO's and is super simple. As Harry said, surface prep is key. Also that stuff requires 5 days to dry I think.

The Graphite does add UV protection, but the best thing you can do for rudders is keep them off the rocks and out of the sun. I forget the ratios off the top of my head, but don't mix more than 7% graphite. Any less than that and the epoxy will yellow. I experimented with taking the graphite epoxy down to 1200 and then buff. It gets shiny just like gelcoat, but it shows all the variations in color. Now I just wetsand to 800 and leave them. I think that's what the factory did. I thought about having a hundred or so foil stickers made up to make them look like NOS. I use 404 high density for filling and 7%(?) graphite. I've done about 20 pairs. It's a lot of work, but worth it.

For your white glass rudders learn to use gelcoat. Take the repair down a little and gelcoat. You've got a potentially nice set of rudders there. Fix them right and look at them as an investment in your sailing career... If fixed correctly they'll last next 20 or so years easy. Gelcoat is an important layer. It is very durable and keeps the water from getting absorbed into the glass (Yes it's porous) and later the core and causing delam.

I think I'll do a gelcoat rudder repair video next.

J