The RV-3 will be non-prepunched, like the RV-6A rear topskin that I recently installed (twice).
Tw key points:
1. the bulkheads must be in line so the skin lies perfect - doing a non-prepunched build requires thinking from the inside out - so, run a string across the flanges of all the bulkheads (front to back) that lie under the skin to confirm that they all line up. Do this at various places. If you cannot move a bulkhead forward or back at this point, and one is out, it can be cut and the height adjusted, and splice plates installed. I had to do this on one bulkhead. See Sam Buchanan's site for a picture of how he did just that.
2. Reinforce the bulkheads with wood sheet. I cut 1/4" plywood to shape, and clecoed the sheets to the bulkheads, particularly close to the flange. A few extra holes in the bulkhead will not matter (I deburred them). Also, run 1 x 2 pieces of wood between the bulkheads at various points to keep them exactly in position relative to each other - you can tape or screw these to the bulkheads. This is why I had to do the skin a second time - I reinforced the bulkheads, but failed to put the pieces of wood between them, with the result that when the strps pulled the skin down, the bulkheads shifted (fore or aft) resulting in misplaced bulkheads and a bent rivet line.
I predrilled the holes in the bulkhead flanges, much easier than marking and drilling the skin, and drilled the skin from the inside, with a helper and a block of wood on the outside puttin in clecos as I drilled. I found that I don't need shims, but thought that I might near the longeron between the skin and the bulkhead flange. If I had needed to shim, I was considering using a "liquid shim" technique, possibly Loctite "Liquid Steel" or something light that. I haven't tried that, but maybe others who have can weigh in. There is some info in the RV-List archives about that.
Bill Brooks
Ottawa, Canada
RV-6A
fuselage in progress