need Ideas - protecting wood outdoors

Spread waterproof glue on the bottom of the feet and then add the
nylon nail in feet. Coat with more than 1 application. This will
prevent the feet from wicking up water.

Buy a can of Rust-Oleum spray rubber and coat the feet.

The big-box stores also sell a protective rubber cap that you can put on the legs of the bench. I would add in a few dabs of silicone into the cap to seal the bottom of the bench leg to the cap.

Can you drill a small hole through the deck under where the feet will go? That'll drain the water.

It's already been painted, but I like the idea of the water seal. Maybe flip it over sand the bottom of the feet and soak. Fear of using the tap on "feet" is the moisture will still be between the wood and the foot. Water Seal sound better. Any other ideas?

Soak the entire thing in tung oil.

Any hardware or home center will have feet for chair and table legs, little HDPE buttons with a nail in the center you tap up into the leg. This will create a small air space and a buffer between the two wood surfaces.

If you want to be 'belt & suspenders' about it you could stand the legs in a little tray full of Thompsons Water Seal or oil of some form. The wood will wick it up and prevent water up-take in the first place.

No water, no rot.

Any hardware store/home center and even Walmart carry nylon 'feet' that just nail into the bottom of the legs. They will keep the bench from sitting directly on the deck.