Waterproofing Spray

[Drax]

In the past I've used Nikwax tent proofing spray to add an extra bit of protection to my Oxford Stormex cover. As I've run out I was going to order some more off fleabay. What I've noticed is a load of people selling Silicone spray.

Question is:

Can you use silicon spray as a waterproofing agent & are there any downsides with using it ie. deterioration of the material it's used on?

I can get silicon spray from work so I don't want to be buying something that I don't have to.

Thanks