Autoglym, water proofing spray
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FYI, Autoglym, seems to work very well the water just beads all over... protects very well.
Looks good, to bad that's not going to stop the water from coming into my/our speedsters.
My 06 Por C4 cabrio came with stock Hartz canvas(?) they recommend Raggtop sealant. used it 1X year, no problems, super beading.
I have tried Raggtop and it is not bad as well I also tired the C2 technique product then I walked into my autosupply local outlet and autoglym was on sale and I was out of the two others that are mail order for me.... so I bought it and so far autoglym seems the best and it seems to come on sale to... so bonus.... my 2 sheckels. 
This stuff works great, too - in the camping/outdoors section of Walmart:
I used Raggtop on my Miata. Its aerosol. It works fine but had to order online. If I had known that I would have gone 303 for fabric tops as it can be brushed on with a sponge brush. I had to mask to prevent over spray. Boat places seem to carry 303 products.
I had to test for colour fastness on my top first and I had no problems with all the products I listed above. I have to protect paint with plastic and tape with the aerosol, I probably would still do it but it might make it easier to apply with a sponge but then again it might leak a bit ... hence the plastic.