It's gonna wear the paint off eventually any way you go. There is a product called Mo-Deck sold at Tractor Supply and maybe other places for the purpose of preventing grass from sticking.

Have a friend that just bought a new mower and coated the deck with a truck bed liner spray, made by Dupli-Color. Will be interested in seeing how this holds up.

Bed liner spray or any other kind of thick coating is a bad idea.

like saxman said any thick coating could only be a bad thing and my personal opinion on the matter is if you are adding an undercoating its probebly to keep grass from sticking so would take a wire brush scrape down the whole inside of the mower deck clean with a rag to remove any residue then spray with a thin coat of primer paint then add a thick coat of automotive high gloss enemel coating . after then dried spray the whole underside of the mower withsilicone spray from snap. i got this idea from snowblowers which sometimes recommend spraying silicone spray on the augers and chute to kee snow from building up. theooreticaly should so the same thing for wet grass and dry grass wopuld stick to the underside of the deck anyway

Any paint coating will chip and wear off. The raw edges retain moisture underneath and promote rust. Why not give
Mo-Deck a try and report back your results with some pictures? Below is a link to Mo-Deck.

Here is a link that might be useful: mo-deck

Ive pesonally used this method with great results :
1.scrape all the rust/loose grass off- needs to be clean.
2.prime rust spots/let dry.
3.use rubberized automotive undercoating, let dry.
4.use Krylon Fusion gloss black paint over the undercoat- it drys to a hard shell like finish.
Another thing ive herd is to use Grain Bin Coating paint for the undersides of decks.
Kidd

Avoid mowing wet grass period! Will make a mess of almost any mower deck! I use good old Prestone Brand from WalMart, silicone spray for less than $2 a can. Spray as much an as often as you want, basically anywhere on the mower, engine included. I do also use a "detailer" spray sold at most bicycle shops called Pro Detailer by Finish Line Products. It's a cleaner, protector of ALL surfaces, with teflon. It's a great product for cleaning and protecting the tops of decks engine shrouds, etc. The down fall is that it's $40 a gallon, so I wouldn't think you'd want to be wasteful by using it under a mower deck. Just my .02

What about WD40?

WD40 - Silicone sprays or any product that leaves a oil film on the deck will not help, it will only make the grass stick more. All it will help with is retarding the rust. Grass will stick (just put some oil on your salad and try to get it out of the bowl) and you have to keep spraying after every use. For snow blowers it most likely will work (water and oil don't mix, basic chemistry)
Two problems - Sticking Grass and Deck Life (rust).
What's needed is a Dry Lubricant you can coat the deck with that protects, lasts, and keeps grass from sticking.
I use a product from Wilmargraphite called CLEAN DECK Graphite-Based Lawn Mower Undercoating. designed just for decks and chutes. Heck at one time even the Air Force used graphite to protect the metal on jets and allow them to reduce friction when flying. Graphite has been used forever as a lubricant. You just clean your deck good, apply a base coat, come back the next day and apply a second coat, go cut the grass. It lasts for me almost all season. I have even put it on other garden tools. It works like magic. Put it on before the season starts. For me a small cost for the product and my time.

Here is a link that might be useful: CLEAN DECK Graphite