I used 303 on the canvas top of my Ferrari Mondial, which had leaked like a sieve, and it worked fine. The top does need to be clean when you put it on, otherwise you will bind in the dirt which is on when you apply it. 303 also make a cleaner. I think 303s are the recommended products for Sunbrella acrylic canvas; virtually all the boat canvas I have ever seen is made of Sunbrella fabric and lots of other things besides. If car tops are washed with only plain water, the waterproofing holds up much longer, which is interesting. These tops are supposedly made with a rubber layer inside under the fabric, so you would think that they would be waterproof regardless of whether they shed water or not, but evidently that is not the case.
I have found that a lot of the products I use to maintain my boat are good on vintage cars and motorcycles as well; waxes and protectants, corrosion inhibitors, and the electrical products used to rewire boats, made by Ancor Inc, are far superior to anything sold on the automotive wiring market. The wire terminals, tinned multistrand wire, fuses, bulbs etc sold by Ancor make auto electrical products look poor by comparison. Their stuff is sold by BoatUS and West Marine- if it can hold together offshore in the salt spray, it will last forever on a car.
