I have been waiting on a shipment from Aircraft Spruce so I can continue on with the wing tips. I decided that I am going to attach the tips with screws, so I needed a bunch of smaller nutplates (I'm using #6 screws so they are a little smaller and won't stand out quite so much). Given the less than perfect accuracy you get when drilling in fiberglass, I bought floating nutplates instead of the standard ones. These mount the same but the threaded portion is able to slide a little in each direction to accommodate screw holes that don't line up perfectly. I match drilled the fiberglass to the holes in the wing skins. The holes in the wing skin are dimpled, which means the fiberglass has to be countersunk. Given the lip on the fiberglass, I couldn't use the countersink cage and had to do it all manually. That's always a recipe for disaster, but no way to get around it. I used a nutplate jig to also drill the rivet holes for the nutplates.
The result is serviceable, but embarrassingly ugly. There's just no way to countersink the thin fiberglass without having the fibers get chewed up and holes turn into ugly shapes. Then of course I misjudged countersinking all of the rivet holes and went too deep for most of them. I used "soft" rivets to attach the nutplates to keep from cracking the fiberglass. The rivets just have to hold the nutplates in position, so they aren't structural.
I attached all of the nutplates and held my breath on the fit. At least the left wing tip seems to fit just fine. While I don't love how close the screw holes ended up to the edge of the fiberglass, I think they'll be okay. The fiberglass is squeezed between the dimpled wing skin and the nutplates, so there really shouldn't be much of a load that would pull the fiberglass in a direction where the screws would want to break through the edge.
