Extra hole
 

If some idiot builder (not me of course) drilled a #40 hole in a skin, in the wrong place, how would he fix it? A dimple and rivet won't work because it needs to be flush on the other side. I thought of filler, but I fear it would loosen and fall out. What do you think?

If the skin is .032 thick, you can machine countersink the hole to accept an NAS1097 AD3 rivet and touch countersink the opposite side, then shaving or sanding the shop head smooth with the skin surface.

If there is something flush to it on the back side then you'll never see it so I'd leave it be.
Jim Sharkey
RV6 - Finishing up