8T,

I use a wax such as Backhouse, but other brands include "Duckback Dressing", "Lincoln - Solid Wax Dressing" or "Barbour Wax Dressing").

My hat doesn't leak, except where the drawstring passes through.
As I'm not too big on "looks", I SLATHER on the wax. Actually, I pre-heat the can of wax in warm water, rub it all over with my fingers, than add a bead all around the stitching/seam where the brim attaches to the cap and where the "walls" of the cap join the top panel.

Then I use a hair dryer on low to melt the wax into the seam. I have a 3/4 Barbour jacket for fall and winter and do the same for it.

If you want, I suppose one could also use SnowSeal and use as described above. Can't find cotton wax at the shop, ... look up "riding apparel" find a good Equestrian center ... Also, any one who sells Barbour products will carry wax treatments.

How much to use, ... well, a can will barely dress a full length riding jacket. On one hat, I'll use about 1/3 of a can! ... If possible, I like to leave it out in the summer sun for a spell, some of the wax will actually drip off at midday.