Advise to use a canvas selaer is frankly just misguided and at best saving a few £'s. Stick to one of the proprietory solutions from BMW or Ragtopp, etc.
Leave tent sealers for tents and roof protectors for the roof.
Abstract from my research and post of a year back:
One thing to note is the 'canvas' is not waterproof and not designed to be.
(I refer only to the automatic roof as the manual roof is an uninsulated construction):
The roof is actually 3 layers. The outer layer is itself 3 layers of poly-acryoniltrite fibres, polyethylene fibres and butyl rubber. Underneath that is a 15mm layer of insulating polyester (sound proof and warmth) and the final inner layer of polyurethane foam and polyethersulfone, for inside finish and insulation. Pretty complex for a folding bit of canvas
My interpretation of this is that water should actually permeate the outer layer and rely on the butyl to be the waterproof layer, so your not really waterproofing it but removing dirt from the outer fibres and protecting them.