I had already started coating the bags. The material is a polyester canvas. I welded the seems together with a flat tipped soldering iron and put zig-zag stitches to add strength. I just cut a long piece of material and made a squared U out of it. I then used two rectangle pieces for the sides and good old velcro for the closures.
I was trying to make drysacks but I had no pattern. I ended up with a small breach in the sides when I roll it over.
So I made a hood to cover the entire top of the bag. Said breach has been addressed.
In playing with Plasti Dip in the past I had always wondered what it would be like if I covered material with it. I figured this was a perfect project to experiment using the stuff. It is very nasty but TOUGH stuff. I use the dip can like paint for the seams and the bottom. I coated the bottom and sides many times by simply painting it on with a brush. I then sprayed the entire bag with the spray version. I kept adding coats until I could not see daylight through it when held up to my work light. It takes about one can of each for the pair of panniers. Please be careful, even when working outside this stuff requires a mask. It is also quite flammable.