I am currently working on this on my 9. My print calls for 426 3-4 rivets. The skin is .040 thick. I have had good luck with a small rubber grommet over the rivet and between the bucking bar and tab on the bulkhead. It takes quite a bit of pressure on the bucking bar to squish the grommet. I take it out after a few hits, once the rivet starts to expand and hold the tab.