Is krylon 100% waterproof?

Probably not very well. The problem is not the waterproofing, however. Lacquers and dopes also shrink silkspan, so the krylon itself will shrink covering on first application. However, it will then partially lock the fibers together, so that subsequent applications will not shrink nearly as much.

Solution is to hold warp into wing when wetting and shrinking. Maybe even put a little extra twist in, as some will relax. Make sure covering is almost wrinkle free before applying dope or Krylon. It will shrink the rest. I'd recommend a coat of clear dope rather than Krylon after water shrinking, then putting Krylon over the clear dope. Test a sample of doped tissue first to make sure that dope is compatible with Krylon, but most are.