Well, you can do anything you want as long as the paint actually sticks. I have cleared over bare metal, I guess I shouldn't say it "can't" be done. But the lifespan on that clear will be short.
I just see it as a waste of time and money. For what the materials cost, I would want them to last more than a couple years. All the "patina" cars that get clear coated on all those "cool" TV shows look good for awhile, and that's all those shops care about. They'll look good long enough for them to sell the car, at which point they don't care anymore. And a couple years later the car will look like crap when the clear fails due to improper prep.
And, that does assume there's no paint reaction between the modern clear and any of the old paint or bodywork. Sometimes it works, sometimes everything bubbles and peels before everything even dries.
If you can't afford to do it right the first time, you definitely won't be able to afford to do it twice. For the OP's car, if he's not ready for a full repaint I'd spot prime those spots and leave it all alone until he can afford to take it to bare metal and paint it, or the bodywork and paint on there now falls off and something has to be done.
