Hi I am new to the forum, I am an ex-coachbuilder now unemployable due to ill heath but I do know my chequer plate or 5 bar tread plate as we called it, I used it day in day out for over 25 years.
My advice if you are painting anything made from aluminium is to use a self etching primer, these are acid based and without this primer you may encounter the paint flaking off in the future, it is available in aerosols if you look hard enough for it. Use a scouring pad (scotchbrite) to key the surface first before priming.
Drilling out the rivets? If they are aluminium they will drill out very easy with a bit of care, if they are steel it’s a lot easier if you punch the centre part (the mandrill) out before attempting to drill. The vast majority of commercial coachbuilding rivets are drilled with a 4.9mm drill and you may find that is what your plates are fitted with, it’s a clearance drill for a 3/16 rivet in old money, I hate metric….
Andy