I've been using the polymax in my drop bags for 2 years now, and really like it. I don't have ammo cans in there, just a bunch of gear under wooden hinged lids. I do usually have a firepan at the bottom of one, but it is in a nylon bag, so wearing the drop bags isn't likely a risk like with ammo cans. I like the polymax on the bottom of the drop bag, because it keeps the bottom of the drop bag flat, so no matter what I cram in there I'm not going to be cramming stuff onto the I-beams of the boat. Sometimes I might end up with a lot of loose stuff in those drop bags, and so the polymax serves as kind of a structural bottom, say if I have a small propane bomb, or something else that might tend to want to hang down in a drop bag, and I don't want stuff touching my floor.
I do know that my rental boat on the grand canyon, all the ammo cans went straight into the drop bags, and those drop bags were bomber. I'm not sure what they were made of, but there's a lot of different products out there so it may depend on what you have also.
Alex, what did you use to "sand" the polymax? Have you tried a surform (small one)/panzer file? They are very fast for smoothing and rounding the corners.
I am a river, babe - I've got plenty of time, I don't know where I'm going, I'm just following the lines..... - "We are water" by Shaye