If you finish the inside before using the vacuum chuck the marks left come off easily with 4/0 steel wool and then you can just rebuff the inside. I have tried lots of things. Commercial grade vacuum foam tubing holds great but has too much flex so the bowls are always mildly off center. I have purchase O rings but then you kind of have to custom size your chuck to fit the O ring. They are kind of hard so don't always seal well on bowls that move a little and become out of round. The Craft foam at Hobby Lobby and Walmart works pretty well but it is also a little thin. It is a closed cell foam so doesn't leak air. I buy the variety without the sticky back and simply glue it one with either contact cement or clear silicone caulk. One reason I like the non sticky variety is that I cut a square bigger than my chuck, then cut a hole in the middle. You can put this piece over the chuck that already has some kind of seal. Put your bowl over this. The square corners make it easy to adjust so you have it roughly centered. This extra piece will almost always seal a bowl that is out of round. It's dense enough not to cause to much misalignment. So for the most part the I like the closed cell foam although most of my chucks still have the O rings. Oh you can cut the O rings and put them back together with clear silicone caulking, so that way they can be custom sized.