That’s true about painting over old paint. Some people say the original paint gives a smooth easier to work with base without all the sanding and prep involved. As say opposed to prepping bare raw metal that is.
As for pricing structure, i haven't a clue.
But I can tell you why they mount the parts to your car before they paint em.
See, color matching is a next to impossible art. A paint job is like a snowflake; no paint job is exactly identical to another. Lots of factors play part in this... room temp, color amount, brands of paint, clear coats, the sun, and so on.
This is why paint shops blend! To blend a paintjob they color match your paint as best they can. Then they paint your parts plus a little of the surrounding area around the part.
Then they thin the color and paint again, this time a little more of the surrounding area. Then they thin the color again and again using a fine mist of spray to finish. The idea is to blend your original color into the freshly painted new color. Side by side the colors are noticeably different, but by blending it deceives your eyes into thinking the two body panels are of the same color when in fact they are not.
You want to know more about painting a car look here:
www.autobodystore.com
This site has helped me a lot.
Good Luck on your quest.
BTW, I’m in the same boat as you. I’m slowly learning how to paint my car.
