As far when your ready to coat the diamond side,get a small can of MEK,Chemical safe gloves and some heavy steel wool and go to town,do all this outside in a well ventilated area.
I like to use a good red oxide primer on things like that,it'll paint over light rust and hold up well...not the cheap rattle can red oxide colored paint,if you have a true red oxide primer,the can will weight much more than any regular primer in comparison.
The fact is on a fab table,,no matter what you use on the underside,whenever you start tacking things to it,welding on the table,plazma cutting..torches..etc etc,the coating on the underside will burn off.
I general leave the table top itself bare,then when i know i won't be using it for days or weeks or months,wipe it all down with vegetable oil or any kind of oil..(vegetable oil being the safest incase you end up having it there when welding or burning as far as fumes),then when i'm ready to use it i'll wipe it all off with acetone or MEK.
As far as when you are ready to weld the frame and legs on the underside,figure out exactly where you will be welding and grind or sand the areas to be welded to fresh bare metal,that'll solve any porosity issues caused by rust.
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