I have the cases, side cover, jug and head of my 1977 dt250 sent out to sand blasting. Tomorrow I'm going to paint them. I'm just curious about any recommendations anyone may have in terms of what paint to use. Should I prime it? It doesnt seem to be primed from the factory. Just tell me your tales.  

Clean clean clean your jug and head numerous times. Make sure all of your jug ports etc are super clean and sand free. I think most members use VHT paint

Side covers may be magnesium.  Not sure how they will hold up to sand blasting? 

Tricky to paint magnesium without special expensive primer.  Normal paint often comes right off if not prepped good.  If original paint is still there you can paint over it.  I have used Gibbs Brand Lubricant on magnesium before painting which sounds like a mistake.  But they say it works and it does.

The link below shows you how the paint simply flakes off the magnesium side covers like Mark just described. Scroll to see how I ended up spraying them with a light coat of Rustoleum truck bed coating. Not recommended if you want the original appearance, but as you can see, my 400 is by no means original. I think the textured finish looks perfect for what I wanted, and it has held up for several years now.
imgur.com/a/CylpdhM
 

Are you sure the covers are magnesium? I had a 400 and they had mag stamped in the side. The left side of this bike is for sure plastic. I believe the right is aluminum. I put some 5% distilled vinegar on it and I saw no bubbles but maybe I need something more acidic? Additionally I got rustoleum engine enamel primer and paint. They didn't have VHT where I went to grab paint. I did an exhaust with this combo once and It came out fine but that was the extra high temp stuff so not sure about this. 

I'm not sure.  I said "may be magnesium".   Earlier ones were for sure. 

5% should bubble up on bare magnesium.

Another test is file the edge and try to light the shavings on fire.

Posting here again because I have painted the parts with high temp automotive rustoleum primer and paint. I made the unfortunate discovery that brake clean just removes the paint. This is not acceptable to have on engine paint. Does anyone have any solutions short of me blasting these parts and starting again? 

Brake clean is nasty stuff.  Not sure there are any engine paints that will hold up to it when the paint is fairly fresh?  Maybe after time and heat has really cured the paint?  Like maybe YEARS?  Even original aged Yamaha engine paint will get hurt by and can be removed by a soak in good brake cleaner.

Really years? I cured the paint in the oven to the specs on the can. I have friends who have painted cases, given using Harley paint, who have been cleaning them with brake cleaner within months, I want to say days but am unsure. I expected "chemically resistant" to hold up to one of the most common garage chemicals but I guess my hopes were too high. The original paint had no issue with me cleaning oil off of it with brake clean. but then again that's 50 year old paint.