Janet, when I bought my new Tarptent a couple of summers ago, I had to seal the seams. (The maker advises customers about this wee chore.)
The sealant recommended by Tarptent is: clear silicone sealer, thinned 2 parts mineral spirits to 1 part silicone, mixed in a tuna tin (or similar), applied with a soft sponge brush.
This was no hassle on my tent: (i) pitch the tent; then (ii) apply the sealant, seal both the inside & outside of each seam, as Dan said, and (iii) let dry thoroughly. Because my tent was new, I also put a couple of swathes of sealant on the inside floor of the tent, to reduce the tendency of mattress and/or sleeping bag to slide around.
Cheers, John