Any 100% silicone will do the job. I use GE Silicone II from Walmart because it's cheap. Mix it 1 part silicone to 4 parts mineral spirits (buy the odorless kind). It takes a lot of stirring to get it to mix, so be patient.
Like Bubba said, pitch the tarp tight to open up the stitch holes. I seal both the top and bottom, just to be sure.
"Life is a Project!"