Silicone spray

Does it have a hydrocarbon base?

I guess it depends on the nature of the chemical. If it is combined with solvents designed to prevent it from curing (which would be my guess if it is intended for moving parts) then it may be a poor choice for treating fabric.

Most of the silicone used to waterproof, bonds or cures in to a solid. What you are describing sounds like it may make a "greasy" film on fabric that would likely just rub off. Silnet and homemade silnet (100% silicone caulk + Alcohol solvent) are curative and turn in to a solid.

Best bet would probably be to test it on a small scrap of silnylon. If you don't have any scrap, let me know and I can send you a few small pieces.

All tarps benefit from water repellant finish, even silnylon. The question is, will your silicone spray 1) adhere; 2) persist; 3) not damage the fabric. The only way to find out is to try it, preferably on a small piece of fabric.

I recently made a bivy out of a Heetsheets emergency bivy and one of those stick-on zippers. I'm looking to seam seal/ waterproof the zipper area, so I'm following this discussion with some interest. Will the silicone help or interfere with zipper function? Will it damage the material the Heetsheets is made out of ?

Stargazer

Silicone spray lube is not the same as silicone sealant and will do nothing except lubricate. A totally different kind of silicone.

Silicone sealant diluted with paint thinner is what works for seam sealing and waterproofing.