I got the CNC cut MDF YZ plates and figured I should paint them before I start attaching parts to them. This unit will end up in a metal workshop building. Humdity in MI is high in the summer.
Any advice on what kind of finish I should apply to them?
Thanks
Jerry
What type of finish do you want? Glossy show quality? Anything but ‘raw wood’?
I’ve had good luck with sanding sealer->primer->paint using acrylic spray paint.
There’s another thread on here about sealing MDF. My comment on that thread was coating the MDF in epoxy and sanding it smooth/flat.
I’ve also used shellac to seal various woods. I can’t say I’ve tried it on MDF.
I’ll be in the same situation soon as I have some MDF parts I’ll be painting as well.
Hammerite goes onto MDF, doesn’t it? Actually, it doesn’t. Technically, it’s a metal paint, not for wood. But there are hammered finish paints that do. You might need to prime with something to seal it, and then plug any holes with tolerances, but it would look at home in a metalworking shop…
For my YZ and Plates, brushed on 2 coats of primer+paint in one, then sprayed couple coats of Shellac. Super smug with the results… Then learned from this forum that Shellac absorbs moisture over time and will haze 
They still look great at 8mths, am in PNW, so not drowning in humidity like some folks here.
So, I have no idea, polyurethane?
Wondering about this too, for higher temp situations like the MP3DP bed support.
I’m just going for protection and then make it black to match-ish the parts I’ve printed.
I’ve been reading about Lacquer as a fast-drying first coat. With the size of these plates a can a spray Lacquer would cover it, maybe even some black lacquer spay paint.
One thing I’ll note. I use gloss acrylic spray paint on the wood I put inside my aquarium stands. The gloss causes water spilled on it to bead and makes it easier to wipe up. I’ll also wax some parts with car wax that I know will be seeing a lot of water.
Probably not necessary for what you’re discussing, but I thought I’d throw that information out there.
I really don’t like the raw cut edge of MDF but you can sort that out with a bit of patience by filling with a bit of dry wall cement and sanding before you prime. A fast-drying first coat is advantageous, and you are correct a spray can of black lacquer (I’d use acrylic) would easily give you enough coats for a nice job. Gloss will show every imperfection - satin will mean less work for you if you want a nice job.
Another side benefit of a hammered finish… 
Another vote from me for hammered finish.
Except that @jtcweb preferred a black finish, for this purpose my preferences would be:either clear gloss (which ironically hides a lot of imperfections!) or hammered finish! I keep four litres of it in stock just in case.
It will actually make the plates look as though they’ve been cut out of solid metal IMHO!