timm wrote:Yeah, that's pretty much the plan. My thinking is that aircraft used to use a similar technique (canvas and dope) and it worked very well. Admittedly dope is a better sealant than exterior paint, but aircraft also move significantly faster than my car. A freak hailstorm would definitely do damage, but a hailstorm would do damage to a teardrop regardless of what the roof was made of, at least my roof can be repaired very quickly and inexpensively.
I can add foam for the reasons you mentioned, but I think the canvas and paint would be ok alone.
I will freely admit that I am nothing close to an aircraft guru, and my knowledge of the aircraft canvas-and-dope technique has been gleaned from the last 10 minutes of scurrying around the Internet. In the end, I agree that canvas and paint would be "ok." From what I can tell, a genuine Grade A Cotton-and-dope covering uses significantly better materials than PMF and is subject to much more stringent QC.
And I have to say: A freak hailstorm shouldn't (within reason) do damage to a properly built camper. A well-built camper is like a little Panzer and can deal with, among other things, bouncing rocks that would crack a glass windshield. And while patching PMF can be a quick and easy fix, if you get rot or mold in the canvas, it'll be anything but. Mold requires moisture, spores, heat and oxygen in order to reproduce. Reducing airflow is the #1 easiest way to prevent mold, and adding a substrate to your PMF will do just that. Yes, patching your PMF skin will be easy at home, on a Saturday afternoon. But if you're out camping on a rainy weekend and a tree branch pokes through the skin, and then you have to deal with a big ol' hole for a weekend in the woods ... bring some duct tape!
It's obvious you're putting a great deal of thought and care into your project. As I said, I'm no aircraft guru, and the unsupported PMF might do just fine. If worse comes to worst, you could always renovate your camper later. Or you could do it for $50 and an afternoon's work the first time :p