Guess you missed that chemistry class
“urethane is hydrophobic, and bonds to the steel surface” …
Hydrophobic means,( in this case), that the material will push away any water it comes in contact with and will not absorb any water in the process. Of course like any body mod, you never do it to a dirty or wet chassis regardless. But what moisture ‘might’ be in your rocker or any box frame part will be pushed away… this assumes that the thermo chemeical reaction hasn’t evaporated all the moisture before hand.