JohnH, I look forward to your upcoming tutorial! Sounds interesting.
I have also had mixed results with waterslide paper and didn't know "not all paper is created equal". Like most things, I guess you get what you pay for... and my first inclination (usually) is to spend the lowest amount possible.
Have you tried a bit of white vinegar in your water?
The last few enclosures I did, I cheated and used vinyl print "paper" then clear coated them. It's easy to use but has its limitations design-wise.
I've also used clear label making material, but again, not a perfect solution as the stuff is waaaay thicker than waterslide.
All those pedals were for my own use, so I wasn't that fussed about getting them perfect and was happy enough with the result. I would feel differently if I was doing one for someone else though.