Aftermarket Seat - Waterproof?

I just got my first aftermarket seat (from Rich's in Kingston WA) in 38 years of BMW riding. It's just swell.

Stock seats have one-piece vinyl covers, which (if not punctured) are impervious to rain. The new seat is also covered in vinyl, but there are two sewn seams, with the resultant series of small needle holes. It doesn't rain a lot in Santa Cruz, and the bike lives in a garage...but I plan a long trip this fall, and I wonder if I should seal the seams (as I might a backpacking tent) or provide a rain cover for the seat when the bike is parked. I don't have any desire for a cover if one isn't needed to keep water off the seat.

What's your experience with this issue?

A quality seat maker will use a plastic barrier between the cover and the seat foam to prevent it from being a sponge. That said, a little water can get to the inside of the vinyl and leach back out.
I've successfully use Sno-proof ( boot waterproofing) on leather and vinyi seats to seal the seams. Treatment seems to last 6-8 weeks. It's a Beeswax product that will leave a nice, nonslippery shine, and has not stained any clothing in my experience.
Spread a layer on the seams & seat, hit with a hair dryer to liquify it, rub it in and let it cool off. Wipe off the excess and ride. Don't melt the vinyl with the heat!

Thanks Bob, I appreciate the info.

Camping stores sell a latex based sealant for canvas that could be used. Nikwax glove proof night also work.