I did forget one other important fact: as long as you store the unused portion of a can of waterproofing spray in a cool, dry, dark space (such as a dry basement), it will keep indefinitely. And it is always better to buy an extra can than to run out mid-way through applying the waterproofing.
I also do recommend that T/O’s be rewaterproofed once a year, as one frequently can’t tell how much of the waterproofing is still intact, until the blanket starts to leak. The cost of rewaterproofing once a year is minor, compared to a horse being strapped into a wet T/O until his body heat dries the blanket lining & fiberfill. Especially since his/her human may not even realize the T/O is leaking.