I'm not sure about "welding" visqueen.

For stake holes I've been using grommet kits you can find at Wal-Mart, and for reinforcing those, and bonding seams, I've used the clear duct tape.

I've used 3.5 mil visqueen, and that's worked pretty darn good. Right now I'm using 2 mil to make a new tarp I'm working on. I think it should hold up pretty good. I'll be testing it soon.