I have both MDF, galvanized covered MDF and plywood. I painted the MDF and ply with gray garage floor/porch paint. I have done transmission work on the ply bench without issues and covered one MDF bench with galvanized for that specific work. Makes cleanup of trans fluid a little easier. I put two coats of paint on and have had no issues on either MDF or ply with a variety of chemicals, including water spills, beer, cokes, fuel, carb cleaner, engine oil, machine oil, WD-40, trans fluid, etc. Porch paint shines right up with a swipe of Berryman's on a rag LOL.
For welding, I would like a steel top bench on wheels but don't have time right now or a place for a 2x4 mobile bench. I have used a piece of commercial countertop for welding. It's not too white anymore, but it's held up surprisingly well.