My bottom-line recommendation is -- don't bother.
I have a JD595 w/54" deck. Tried a graphite-based undercoating for 2 seasons. No real benefits. And it demonstrated for me pretty convincing evidence that coatings are pre-destined to have little effect. My experience is that mowing always throws up many abrasive things in addition to the grass -- sand, pebbles, rocks, branches, etc. These agents will wear down and remove your coating/agent from those places of your deck that receive the most flow. And the grass will continue to build up on any studs, nuts, or other protrusions on the deck underside.
The best approach is to mow when the conditions are at max dryness. Even then, you'lll have to peridically get under the deck and clear out the build-up. No easy way.
jim