I'm not sure if I'm going to bother blending the edges of the fiberglass tips into the aluminum or not. Some tips fit nicely and don't need it, but most have all sorts of puckers or gaps between the rivets. Once approach is to completely cover the joint with light weight fiberglass cloth (to keep the paint from cracking - kind of like taping a drywall joint) and then put micro on top of that to blend it into the aluminum skin. The other option is to simply build up the fiberglass itself to match the aluminum edge, but not extend any fiberglass over the actual joint space. I'll probably do a combination of that, just depending on the part.
The rudder cap actually fits pretty well, but I figured it was a good spot to experiment on. It's way up in the air, so if I mess it up, it's at least not super visible. I scuffed up the tip and about an inch and a half of the aluminum skin, then laid down one layer of 2 oz fiberglass cloth. This is super light and thin compared to the normal cloth used for everything else (usually 8-9 oz). I put peel ply over that and let it set up. The peel ply is a thin dacron cloth - when the epoxy dries, you rip off the peel ply. It pulls off the top, glossy layer of epoxy and leaves a slightly textured finish. If you use this, you don't have to scuff before putting more epoxy down.
After pulling the peel ply off, I put a thick layer of micro down. I wasn't sure how far up the cap I needed to go in order to feather it, so I ended up putting micro on most of the cap and down to the edge of the cloth. Once that all dried, I started block sanding to feather all of the edges. I think that's the learning curve part. Some areas sanded down to the aluminum because they were higher than the surrounding areas. I guess in the end with paint on, what matters is that it's all level and smooth. After an hour of sanding it was still a little wavy. My sanding blocks are all pretty stiff, which make contours a little tough to make smooth. I'll have to run to town and see what random stuff I can find that would work as sanding backers. I need some softer, round tubes like a pool noodle or pipe insulation, and some stiff-ish, but flexible hard rubber for flatter (but not flat) surfaces.
Once I'm happy with the finish and shape, the next step will be to wipe on a couple of thin layers of straight epoxy to fill the pinholes. Micro is essentially little glass balloons, so when you sand it, you end up getting pinholes either in the balloons themselves or between them. After that, I'll sand to about 400, then spray a light layer of epoxy primer to seal it all. That will also help show any remaining pinholes. One it's time to paint, the primer will be scuffed to 400 and then a normal layer of new primer laid down over it.
