TOPIC:

I've started stripping the frame on my '79 KZ400H LTD and noticed there is no primer under the factory black paint. After exploring a lot of paint options, I'm thinking of just going with Rustoleum Satin Black, which I've used before on bike frames with primer and got good results, but I'm wondering what the benefit would be to use primer this time around. Primer is mostly used to give the topcoat an adhesion surface, and I'm not trying to build this bike to Concourse specs. Not using primer would save me a lot of sanding, but I think it would be more prone to scratching...I guess ultimately it's up to me, but I'm looking for opinions here. I also might topcoat it with some Clear, but I'm not sure yet.  I recently used some SprayMax 2K Clear on the tank and side panels of the '76 KZ400 I did, and that stuff is expensive, but worth every penny! Easy to use and it dries brilliantly clear and hard, like a 2K should.

Anyway - any opinions? 

What does the paint manufacturer recommend ?? 

You could have it done with a satin black ceramic coating.

Drop the frame off, shop does the media blasting in no time at all (compared to sanding), they apply and bake the coating, and it's done. The VIN sticker on the steering head probably wouldn't survive the oven (so the shop will likely remove it) but as long as the VIN stamping is still visible you'll be fine. If you have other stuff to do in the same finish, you can do it all at once and get a better deal.

Some like to powder coat the frame but most of what I have seen appears to be too glossy.

I have found that a good epoxy primer will not only seal all moisture out (which is paints enemy). It also sands MUCH EASIER and cleaner than paint.   Paint is the hard candy shell but its thin. A high build primer is thicker to cover imperfections, but is a little softer which helps it sand nice.
If you don't want to spray it yourself find a powdercoater nearby. They have multiple clear finishes to bring down the powder"shine" you get. 
my thoughts are only an opinion, hope it helps.

The paint that Kawasaki used was much more durable than rattle can Rustolium.  I'd hit the frame with some primer first.  Paying someone for powder coat is 100 times more durable, but also much more expensive.

And for what it's worth, on my 750 frame, Kawasaki used a yellow primer on the lower sections of the frame, but none on the upper sections.  Maybe they were thinking that road debris will hit the lower half of the frame, thus more protection will be needed?