OK - go for broke on the waterproofing. At $3.97 a can, I used almost a whole can on this one loaf-pan size hide. The more waterproof, the longer it will last. get in any nooks and crannies really, really good. Be sure the final coat over the Instant Stone is good and thick, and since this is not an art project (where you are not worried about yellowing or it being too thick for looks) make sure you get a good coating over the Instant Stone! So you can wash it. I did not waste Instant Stone @ $8 a can underneath, only on the outside. Also, let it air out for a few days outside to get rid of the spray paint smell.