Im getting ready to begin my maccat clone diy tarp and about to seam the ridgeline. In order to get a really straight line with this very slippery material I want to pin a few inches in from where I'll be seaming so I wont have to pull the pins as I pass the fabric through the machine in addition to pinning the edge to keep the fabric together. I'll basically have 2 tracks of pins, 1 of which I remove as I go, the other spaced out away from the thread injector keeping the material from sliding.
My concern is: will I have water leaks where i've pinned the fabric due to this excessive pinning?
If so do you guys recommend a better method of keeping the fabric straight on this really important first stitch. Any tricks you guys could pass along to help a noob sew a straight flat felled french seam will be taken to heart.