Why cars rust

not to get off topic but was thinking of spraying underside of lawnmower deck with some type of undercoating because that seems to go wrong with most mowers arond fla the decks rust out

iggys.... that is because 1.you are near salt water 2. as a lawnmower actually does it s job the paint is essentially "whipped" off of the underside of the deck exposing bare metal....bare metal & salt water atmosphere=rust
A friend in Michigan has all his NEW mowers media blasted on the underside then spray coats them with POR15, he does the same for his snowblowers..... he has replaced several motors on his machines ( he has like 25 of each its his business) but rarely replaces the decks or the shoots.....every 2 -3 years he media blasts them and re-coats them.Nothing stops rust but he seems to have slowed it down to an acceptable rate