Washed mine both sides with scrub brush and laundry soap. Rinsed very well then dried completely, not even damp.. Used the spray stuff from Canadian Tire, cant remember name but not real expensive, I tried cheap before going to something "expensive", cheap worked as it was 3 years ago, still no leaks. Do suggest spray until wet, then put back on boat, it will shrink a small amount and I did in hot August sun, being on boat saves any reinstall hassle. Good luck, was an easy afternoon.
HM