When I made the fittings I used 3/16" pilot holes for the 5/16" bolt holes at the ends where the wing flying and landing wires attach, as well as for the 3/8" spar bolt holes at the other ends. I didn't drill them to finished size at that point because I didn't want to buy large bolts to ruin during welding and I wanted to use small cheap pieces of tubing for the spacers. The drilling was no more difficult after welding then before. I drilled them out with center drills because the steep angle on the cutting edge helps them center better on the pilot hole. The 3/8" hole was then drilled slightly (.002") larger with a letter V drill.
I've decided to go back to using Zinc Chromate for primer except where I need to protect the finish for applying fabric. There are wood blocks around these fittings for attaching the fabric so they're well protected. The epoxy primer is much more easily scratched than Zinc Chromate.
I finished them with Black enamel, for the same reason, and then baked them in the sun. I still need to paint all the small fittings and form the drag and anti-drag wires.
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