I use it:
1. bubbles up for laying out my project
2. bubbles up for the Rubbing Stage
3. bubbles up (sometimes) for spot-sanding
4. some rolling if the design is complicated or multi-layered.
I also use it as a "cushion" (smooth side up) when I do all my sanding ----and this is without any towel underneath!!! (more on that in another post)
So the stuff does eventually wear out. I do work it to death.......but when see it start to "buckle" unevenly or the bubbles feel "sharp to the touch" or fibers are sticking to the bubble side because they get snagged on the rough edges................time to replace it.
I have always ordered mine from http://poolsupplies.com in Tonawanda NY (near where I grew up) because they still sell (thank heavens) the Clear Round Bubble 16mil thick Solar Pool Cover in a 4' by 8' sheet which is the perfect size for 90% of my projects. When it comes, I cut it in half lengthwise and use the 2 pieces until I wear them out again.
For the old covers I cut them up into smaller pieces so that I can do smaller projects and use them for all kinds of spot-sanding and dozens of other tricks.
The Cover is $24.95 but you do have to pay $10.95 handling (shipping free) because, as you will see when it arrives at your door, it looks like a rolled up rug.........you are paying for the odd over-size.