what is the difference between

Acid etching primer is great for bare metal. It uses an acid additive that microscopically eats into the metal surface and gives a very tough bond between paint and steel. Only a light coat is needed, followed by a sanding coat of regular primer. Regular primer is used to build the surface so block sanding will result in a perfect base for final paint. It acts as an overall filler to help level the surface and is designed to bond well to underlying surfaces and overlying coats of paint. It isn't intended to be a final finish so will absorb water, degrade and chalk of if not protected. Cars with a nostalgia 'suede' or primer finish should be clear coated with flat urethane clear coat.

thanks Willys36.that makes it a little more clear to me now.the last car I painted was too many years ago to remember. That and most of the paint was still lead based,

Yes, that wold have been a few decades ago!!

When I did corrosion control for Uncle Sugars Flying Circus, the instructors told us time and again, "If you draw a pencil line across the surface and you can feel the indentation, you put on too much!"

Madd - is that the standard for the acid etch? makes sense, all you need is a covering coat of the etching primer then use the filler primers for the buildup to block sand.

well I do not know about standards on acid primer but it sounds good to me.Now if you want to know stereo systems or engines then I might be able to help but painting I am learning all over again.
Lots of new thing to try.