Bill has told me, the past few years his lawn mowers had problems with the decks, they rusted out. In fact, the Brigg and Stratton motors lasted forever. It was the decks that went to pot. He just got a new lawn mower and was thinking about using Rustoleum undercoating. But should he cover the blade first with something before he sprays on the undercoat protection

I prefer brushed on paint, (Enamel, do NOT use house paint). Clean the deck well and wash it out at the end of every season, several times a season or as much as every use if its left outside or in a wet environment.

Undercoating can be an ugly material to deal with once its applied, rocks can be kicked up exposing bare metal, paint is easier to sand those and touch up than gooey undercoating.

Also if you run the tank dry until the engine dies on its own the end of every season, you will have fewer hard starting issues every year.

Maybe he shouldn't do it, to the new mower, it is only a month old.
He already purchased the spray can of undercoating, but he could bring that back to the store. Thanks for the advice

Perfect time to brush on paint, is now BEFORE it starts rusting. But yeah, he can take the undercoating back.

Another important point I failed to think to mention...undercoating is soft, too soft, even grass will be hitting the deck hard much less twigs, sand, small rocks etc.....paint will be more durable.