Finding a waterproofer for Tarptents and other items proved a bit of a challenge in itself. You can get Grangers and Nikwax but they don’t work on Silicone impregnated nylons.
Then i found this product from McNett, called Thunder Shield, which i bought from Advanced Elements Kayaks, here.
Apart from just Silicone products it will do some waterproofs (but not Goretex and similar breathable ones), tarps, rucksacks, tents, chest pouches and more.
Ordering from AEK was fantastic service, arrived next day.