Have had to repaint a number of times after workers have thinned out really good and expensive paint when I wasn’t supervising. It’s maddening. On metal, rust will win out if not full coated in this humid climate.
For your floor, If you’ve sanded or otherwise prepped the old or new wood deck well before, thinning is okay for first but not really necessary.
I put 5 full coats of Begar Polyurethane 1k Suoreme on my deck with light sanding after 1, 2, and 4th coats. (2 coats on bottom of raised deck). Deck was made out of handpicked salvaged teak.
I’m having to do most of the painting myself to get a decent job. Only a couple of guys followed my directions on how to brush without leaving brush strokes, feathering instead. Unfortunately, both were alcoholic and didn’t last, even after paying and helping one into rehab. Very sad situation here in Thailand.
Sorry for the sob story! God bless them all.