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I have previously used primer and canned spray paint on projects made from mild steel. Works well, but what are the other options? I have made a metal bench and would rather the top have some kind of protective coating or wax to stop the rust, but paint will scratch off quite quick - and probably bubble as doubtless I will end up welding the odd thing on it.

For other projects I have, is it possible to powder coat at home?

You can powder coat at home but the equipment isn't cheap and of course you need an oven bigger than the item you want to powder coat.

Metal work bench?
Depending on your use, i'd be inclined to leave the top bare steel if you plan on using it for welding, just attach the return (earth) cable to the top and you can weld whatever you put on it without having to keep attaching the return cable to little bits.
Just keep a light coating of oil on it. Or even cheapo type wood wax will do. (Assuming its not outside all the time) Over time it may just get a light surface rust, but just run a bit of scotchbrite or sandpaper over it and its clean again.

There is loads of options. Powder coat isnt all that DIY friendly really.
Aerosol can paint can tent to be a little thin. Get a litre of good old zinc rich primer, then use whatever top coat you like.