I used the Polymarine stuff on my old SR - worked wonders, but as said above, make sure you rinse it properly before it dries! As already mentioned, bl**dy expensive, but the 500ml spray thing I got did an entire SR4 (including the hypalon foredeck) and I still have half the bottle left.
As for the current boat - once I've finished peeling off the previous paint job I'll need to decide the best course of action......
Remind me to stop buying old RIbs.......