With the Membrane in place, I focused on the bottom side of the
Slosh Tubz They come with a very smooth gel coat on top and a slightly rough fiberglass texture on the bottom. Its not necessary to paint them, but I wanted everything to match. My buddy Yuma painted and buffed the top side, now its was my turn to finish out the bottom side.
I like to coat the bottom of the Tubz with a heavy duty rubberized undercoating. I use Medallion brand on all my fender liners. It gives a thick layer of protection from road debris and looks great It also adds some sound dampening as well.
I masked the front side to protect the fresh paint, then sanded the bottom side. I'm not after a smooth surface, just enough to knock the fiberglass nibs down some. Once that process was finished I wiped them down with brake cleaner.
The undercoating requires a Shutz gun to apply, You can get them cheap at Harbor Freight. Be sure to clean it thoroughly when done, or it becomes disposable. Find an area to test your trigger pull and air pressure. I prefer high pressure with a light touch on the trigger to get a large texture pattern.
Apply the product in several coats changing directions to cover the textured surface. Once you have full coverage, I like to add a coarse top coat for appearance purposes. The end result is a surface that can be cleaned easily and looks great.