JJ, if the marks you mention are wearmarks, then cleaning and reproofing probably won't remove these.
However, a deep clean and re-proof every year makes absolute sense as it will extend the life of a hood significantly. As we know, replacing a Boxster roof is expensive. If you take the roof down often, keep the car outdoors in all weathers and don't maintain a convertible roof regularly, then it's not unknown to have to fit a replacement hood in as little as 7 years.
I've used Autoglym, Renovo, Scotchgard and BMW OEM convertible cleaners. In my experience (I've had convertibles for a total of 10 years and have run a car valeting company) Autoglym is the easiest to apply, Renovo is the most water-resistant (none are acutally waterproof) and longest-lasting. Renovo is available for both black and blue fabrics. In my view, Scotchgard is a pain as it is supplied as an aerosol and goes everywhere. The Porsche OEM kit is vastly overpriced. Renovo is what the valeting trade tends to use, BTW.
Neither Fabsil nor Nikwax are marketed for vehicle applications so personally I'm dubious about using them on cabrio roofs.
As Nick says, using a decent vacuum cleaner first to remove embedded road-dust,volcano fallout, bird dropping residue makes good sense. The biggest issues for me are:
- waiting for the roof to dry after cleaning
- avoiding getting any of the proofers on the glass or paintwork
- weather
Valeters overcome these by using a garden blower to speed up the drying process, using a plastic dust sheet (£3.99 from B&Q) with a hole cut in it and masking tape to cover the paintwork, trim and glass (see attached picture) and using a portable shelter on-site or working indoors.
Finally, I seem to recall somewhere in the small print manufacturers usually recommend that car shampoo isn't used on the hood fabric whether new or after reproofing. The strong detergent used in most car shampoo formulations acts as a 'wetting' agent and reduces the ability of the proofer to work.
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