I think Vans will tell you that the countersunk holes will be just fine. The below was shamelessly stolen from an old post.
Excerpt from Vans FAQ:
"For AD3 rivets, .032 is the minimum allowable skin thickness [for countersink], and for AD4 rivets, .050 is the minimum thickness. Metal thicker than this is difficult to dimple, so it is common practice to machine countersink for AD4 rivets.
While it is acceptable practice to machine countersink metal as thin as 0.032” for an AN426AD3 (3/32”) rivet, we recommend dimpling in metal 0.040 and thinner. There are a couple of reasons. First, although RVs are designed so that a countersunk joint is acceptable, the interlocking nature of a dimpled rivet line is stronger. Second, dimpling leaves a thicker edge for the shop head of the rivet to form against, meaning that the metal deforms less and there is less chance of “working” rivets later.
This recommendation is meant as a guideline, not an absolute rule. There may be instances where countersinking in 0.032” stock makes the job easier, and certainly can be used. "
RV-10 - 950 hrs
First Flight Oct 2012
Aviation Partners, LLC