Cakey is absolutely correct about the best solution being a thorough cleaning immediately after wet mowing. This is a very practical approach for some mowers that have easy access to the bottom of the mower deck. For many midmount ZTR's, however, this can be quite difficult, especially for those having trouble physically getting underneath.
This is why I REALLY wish that my Grasshopper had the power deck. I have a floor jack that raises the deck relatively easily, but my mid mount Bad Boy is only accessible by lying on my side with a piece of metal strap as a scraping tool.
The undercoating might work good, but I would think that the surface would have to be Spic & Span clean in order for it to stick.
Larry