I had my '65 done years ago. Now I want to remove the wheel lip and rocker moldings. Although it is still in pretty good shape, the paint job is also starting to show it's age. The local body/paint guy says that after welding the screw holes from the removed trim and finishing them off, that he only needs to 600 the rest of the car before painting because the existing paint has a good bond. Does this sound right or does the car need to be primed/sealed before paint. I only want to do this once.

That is correct.

Many have successfully repainted after they washed the car well, wiped with wax and grease remover, welded, patched, primed and finished the spots, wet sanded the whole car to give the new paint some tooth and to blend with the spot primed areas, sealed the whole car prior to repaint.

Should be fine,but it really depends if you want a good quality job that lasts long time,do you know how many times it has been painted? How thick is the existing paint? That is what you'll have to live with if you paint over old paint.Not really knowing what's underneath can create problems down the road, but if you are on a budget and don't want to spend alot of money,just sand it down and shoot it.Chances are if old paint is OK and there are no cracking,checking,peeling,or blistering,you should be fine. But if you want results of high-end paint job, then you would need to strip it all down and start from scratch.

A good painter can tell if you have too much product on already & lots of shops have mil gauges, too. If it's not too thick & no lifting or crazing you should be fine but, I personally would put a thin coat of sealer on the whole thing for bond & color consistency.