The reason we give more attention to the areas that are to be cut is because those areas will have an edge where the sand and air are trying to get under the tape. We want to make sure the tape doesn't give in to this pressure. If it does, we'd get overblast which is a bad thing. Another nice thing about contact paper is that it can be manipulated to protect just about any shape. Granted the spherical shapes may take some additional work since the material is not that elastic, but with enough practice you can get it to work.
Contact paper is our commonplace fixall solution.
We consider it to be kind of like duct tape. It can be found almost anywhere, and can be used for just about anything. The fact that it can be found almost anywhere is a valuable asset all in itself. Shoot on down to Longs and get a roll of scottish plaid. It's untraditional, but it works.