Thanks but I can't find a camera at the moment. Its a normal "felled" seam, sewn, then folded over and sewn again. The seam on the outside of the fold is the problem, that is where holes appear.
Jerry and Jim
I'd forgotten about that article. I think I'll try gluing a strip on top first. It solves the problem an if it doesn't work I can try something else.
Thanks guys, I've got some ideas to work with. If I feel like its going to be more trouble then its worth then I may use half of this project for a bivy bottom and the other half for a pair of rain pants. These are things I actually need more then a new tarp at the moment so we'll see what I end up with.