try some of the big camping shops, I'm sure they will know what to use, have never used a product like that but I do know they are out there.

I always waterproof the seams

In the canvas by rubbing Bees wax

Along the seam stops the seam leaking

But never tried going further

Used to be able to get a spray for\

Waterproofing motorcycle jackets ect

Most sports goods shops sell that

We made our own annexe walls for our Aliner - used a lightweight un-seasoned canvas. Once made, each wall was laid out on a large table in the backyard and painted with a thin coat of COI Leisure 'Aqua Proof' (Made by Primus and found in most large camping stores), we use a normal 100mm wide household paint brush. Aqua Proof is suitable only for weathered canvas. The trick seems to be to apply a thin coat evenly over the whole surface. Initially the coating has a whitish milky tint to the canvas, but dries clear. Our 2 litre container is about half full, and has done us nearly two years now - we re-coat about twice a year. The annexe has weathered some significant storms over the last two years.

Regards, Brian