I used the spray cans from O'Reilly Auto Parts, and I used a LOT of them. The spray can stuff goes on pretty thin, and requires many coats to get anything that resembles undercoating... which equates to a LOT of cans.
I think I used something like 11 or 12 just for the floor pans, intermediate area, inside wheel houses, and trunk pan. It also still looks like I used a bunch of spray cans under there as well (fair warning).
This is after about a half dozen spray cans, and before I pulled the rear axle for its own rehab. I actually sprayed the whole thing down again before I put it all back together.