I'm not sure that this is a modification that you should be considering based upon "quick" advice...
What is used on coats and such may not be suitable for a parachute.
I have a bottle of liquid seam sealer used to waterproof seams on camping tents, but I'm not going to go using that on my parachute without long research and expert advice.
Worry not, I wouldn't going anywhere near a parachute with this kind of thing. This question relates strictly to non-flying things such as waterproof coats/tents/bags etc. I'm asking here as I'm sure some people will have some knowledge of appropriate products (brands, products, retailers etc).