About 2 years ago, I would have said POR15, but no more. Stuff was applied correctly and still comes off. Don't put anything...and make a habit of washing it out with a lawn sprinkler after each use...then at end of season when you are winterizing spray the underside with Fluid Film, Rust Check, Krown, whatever and sleep comfy until Spring arrives again.
I just added a garden hose connector on the deck like the new fancy mowers have, and use it after a wet coleslaw mow. If it was dryish I don't bother. Is your parking spot for it dry? I think that's the main thing that is doesn't sit with wet grass in it for a week.
I have tried about everything including graphite coatings. The thing that works best is Rustoleum Professional High Performance spray paint. It is industrial quality and holds up pretty well. Also it dries quickly and after a couple days you can mow with it. Every fall I scrape my mower clean and use it. You can get it at Home Depot and Lowes. Make sure it is the Professional High Performance in the 15 ounce cans.
I would not use anything but a decent metal primer and a couple coats of gloss top coating. Rustolium or one of their competitors it does not matter. Nothing thicker that could trap moisture when the finish gets hit by a rock. The gloss has better nonstick properties and gloss paint is harder. It will help for a few years but eventually it will be so scared from rocks and scraping the thick grass it wont be much help anymore. The best thing you can do is keep it inside either a shed or garage NOT under a tarp.
When I bought a larger motorcycle I had to make room in the shed and the lawn mower went outside. It was 18 years old and looked and functioned almost like new, 1 year under the tarp and it had rust everywhere and started giving me trouble starting.
I also use Rustolium Professional High Performance spray, first the primer then the top coat. It holds up well, but not for ever. I tried many things over the years. I was once going to try epoxy coating for boat hulls, but it's expensive and it may end up cracking off anyway.
Grass Shield, you can buy it by the gallon jug at Gemplers.com. Put it in an old spray bottle like Windex and spray it on a clean deck. I mow about 5 acres and I still have a quarter gallon after 3 years. I spray it in my snowblower chute as well. It's nice when you find a product that does what it says it will do.
I would use the Rustoleum on it. Over time anything you apply is going to deteriorate with all the debris flying around under the deck. Then cleanliness is you next best bet. I have found to run the mower through a small leaf pile in the fall. That will scrub all the stuck on junk from the underside of the deck without hurting the painted surface to applied..