I used a flux brush (I have extra) to paint the bottom of my B-17 with Proofhide. Got it on heavey enough that way, that I'll likely never have to do it again, along with excellent coverage.

The tin will likely last me many, many, many years.

With all the rain we have gotten this year, I still only do the top annually (in the spring-when it will stay "wet" for a few days.)