I have not had any problem with crushing using 1/16" Balsa skins with the vacuum pressure regulated down to 10" Hg. My rig has a pressure control circuit but you can regulate the bag with a needle valve and vent, you do need a vac. guage though. A pressure even lower than this seems O.K. down to about 6" Hg. This relates to 3 lbs/sq.in. which is an awful heavy weight on a typical wing. A good tip if using balsa, or anything that is a bit porous is to seal the inner surface first. I use a thin shellac solution which adds nothing to the weight and it dries super fast as it is alcohol based. I know guys who have't done this a nd resulted in a dry joint with the skin coming adrift with a bit of "G". the resin is sucked into the Balsa under Vacuum, leaving nothing for the joint.