[DWThomas]

Ah I see, so you dimple it rather than pierce it and then when you sand the back of it the dimple gets sanded through before the rest of the material? I think some of the acid etched pinholes are dimpled around the actual pinhole rather than just a 90 degree hole as is with the laser etched ones which apparently gives a clearer image.

Yes, think of running a razor over a zit -- uh -- on second thought, maybe we'd rather not!

From my ancient days in etched parts, circuit boards, etc. it's almost inevitable that there is a slope to the etched edge, so that could produce a bit of a knife edge, even in a flat sheet.