On boats (and I grew up working on the back of a lobster boat and building boats for a long time) the gel coat is put direct to the laid fiberglass, can be any color, and is a supper tough paint. that is why most do not want to remove it from boats, very costly to be redone, but can be touched up if removed.
on vets, there is no gel coat from the factory, usually its a black or dark gray coating that looks like primer, but is a little hard to get off.
it seals the fibers and keeps the mold release agents from coming back out. (the new vets 1970"s? and up have it mixed right into the fiberglass so it can be sprayed on a production line. if blasted or sanded it will keep coming out no mater how much you take off.
on the older vet pre 70"s? the cars were hand molded and a release agent was sprayed on to the mold, then the fiberglass laid in. (like boats are made.)
if there is a gel coat on a car then it was put on buy a previous owner.
most times, boats are painted over the gel coat with a paint call Awl Grip. anther tough to get off paint, usually the top sides of boats are done with this, but not the underside were the bottom paint goes. the bottom usually gets a barrio coat (like a rubber) then its painted over.
when blasting vets, if you can get the paint off down to the black or gray "last Coat" then its usually good, they don't need to be bare like metal.
if you go to far, and see the fibers of the fiberglass, the body shops just has to apply the first coat of sealer (not primer) to build the paint up.
if they give you hell about going to far, then they are just Bitching because they have to do more work.
I to work for over 10 HIGH end body shops that do $100,000+ street rods, and rebuild for customers....
and a lot of other body shops that do regular normal stuff.