Ok I know this is supposed to be an old family recipe handed down, and supposed to be kept secret. But here goes. Nothing works forever, but the best substance that I have found is to melt an entire block of sealing wax or parafin (for canning), add a cup of 30 wt motor oil. Pour oil into wax slowly for safeties sake. Using a horse hair brush not nylon. While hot paint the concoction on all exposed surfaces under the deck, and allow to cool overnite. Rememeber that all the exposed areas must be really clean before applying. First I powerwash then I use Castol purple power cleaner at 1/2 ratio let soak a few minutes then powerwash again. As I said nothing lasts forever but this concoction will last 6 to 8 weeks depending on how much or sand dirt your mower picks up. Most grass won't stick or clog and if it does it rinses out with just a garden hoe and nozzle.
P.S. Don't tell my dad I told you.