In reply to tingle:
I have used a product called Fabsil gold by grangers. It comes in a 5 ltr metal tin and you paint it onto the outside of the tent with a paint brush while the tent is erected. Leave to dry for 8 hours and should be water proof after that? The stuff you apply with a brush is far better than the spray can products.
I have used it on my fishing bivvies and shelters and also many coats and it has been brilliant. They also make seam sealant which is a bit like silicone. This is applied over all stitched seams and left to dry. It does not look very nice but is very effective
I kept the same fishing bivvy water tight for 10 years with this product. The only downside is the treatment removes any breathability from the fabric so on single skin tents you will get condensation
Post edited at 10:19