Altogether between my wife & I we have owned 4 soft tops (2 Boxster's, Audi TT, Audi A4 Cabriolet). They all remained immaculate throughout our ownership & I've never used a brush to clean the top of any of them (I do all of the cleaning etc). I mainly just wash the cars & dry off the top with a good quality synthetic chamois. If that's done regularly & the car is garaged then I haven't found the tops require a great deal of maintenance. Any marks usually come off pretty easily, but on the very rare occasions they do not, I use Autoglym convertible soft top cleaner. This is available in a clean & protect kit along with a protector/re-waterproofer & is supplied with a special applicator sponge (denser than a regular sponge). I don't use the re-waterproofer more than once a year & have never needed to. I personally don't think using a brush on your soft top is a good idea. You might get away with it, but it may scuff up the material & mark it & I'd advise you to be extremely wary of that approach (it's likely to alter the texture of the material over time even if it didn't appear to damage it on first use). Autoglym & the makers of the product I used prior to that (sorry the name escapes me) both recommended a sponge & also provided one with their products, that was good enough for me & I've stuck with it.