Originally Posted by JackD

It is obvious that you should only use aircraft grade aluminum for shims. If you insist on using soda cans, flatten the material between two granite surfacing plates and use #1000 grit to make sure it is consistant across both directions (the cold drawing process of constructing cans can lead to irregularity in thickness). Following that an anneal and quench cycle will relieve any latent stress. Once you have that done make sure to round the edges to remove any possible stress risers. And finally, prepare a steel mandrel 0.008" in diameter less than the bars to bend the shim around. If you bend around the bar, the material will naturally spring open a bit larger. DO NOT bend with your fingers.

Powder coating the shim while attractive is a BAD idea as it changes the coefficient of friction and totally f's up the hard work you have done dimensioning the shim.