So I'll get into a little bit of the non-vanilla variables that may influence the outcome.
1) Thin, pliable and un-supported substrate
2) Need re-use the substrate unmodified (including the original pilot holes)
3) Heavily corroded rivets and rivet posts
Couple of the things I tried
1) HSS 118* and HS CO 135* bit types
2) Bit sizes 1/3 (3mm) and 15/64 (6mm)
3) Straight drill, no punch
4) Carbide Centre punch + Drill
5) External support for the substrate (eg, wood blocks etc)
The main issue/problem in my opinion that was encountered
A) Not removing or flatening-square the centre post forces off-centre drilling
A2) Also forces a centre punch off-centre
B) Off-centre drilling increases risk of figure 8 hole vs orginal clean pilot holes
C) Rough-shape posts and corroded posts were primary culprit
Things I didn't try, which I would consider based on this thread feedback
6) Spot grind or File the post flat before drilling (more square= deflection less likely)
7) drift punch out the post, not centre punch (which just deflects)
8) Using an even larger (1/4) drill or centre-drill bit to drill (stiffer, less wander)
