Werner,
Yeah, Thomson's clear waterseal works great on acrylic and cotton fabrics. They make a version for "tropical" environments that has extra UV and mildew protection, too. Make sure to get the clear variety; they have colored versions that would wreck havoc on your deck.
I always suggest using this stuff to folks who have me make their sail covers. It's a LOT cheaper than 3M 303. Also, camping supply stores sell waterproofer for nylon tents; it's a bit more expensive but does the trick.
Cheers,
Aaron N.