Can I take a minute and try to put this all in some sort of context?
First Matt asked:
"Inexperience question here.
My understanding regarding backing (baking?) is that it is intended to drive off the volatiles in the freshly applied paint. Is this correct?"
Matt, yes it accelerates drying process by evaporation of the chemical drying agents in the paint, Jerry Glow could explain it better as he is a former auto body repairman. The same baking techniques are used to do the same to prototype freight equipment, it is the application that differs.
"Also, I believe I read somewhere that letting a freshly painted model sit for several days (or longer) would accomplish the same thing - only (obviously) slower...
That could be true but the heating process makes the enamel dry harder quicker. As I have said I don't bake my brass in the oven but I do in natural sunlight under supervision or under a 60 watt bulb perhaps overnight in winter. If I want to bake solvent based enamels on plastic I dry them in natural sunlight with extra supervision as the combination of the solvents and the heat can soften plastic
The Peter adds:
" I had been lead to believe that baking paint like Scalecoat, would actually improve the adhesion of the paint to brass."
Not really, adhesion would improve if you do as Schuyler and Mike mentions by lightly etching the surface chemically or with "grit blasting"
"I know from experience that baked on lacquer and enamel paint resists scratching better.
True" enamel" by it's nature/name is expected to be harder, the same with lacquer-based enamel, which would leave one to believe it would resist scratching better.
Baking is only appropriate for enamel based paint."
Wait! I have an issue with this statement and it follows along with the use of the term, "Dulux Gold", Dulux is a brand not a specific color and enamel is referenced to a hardened surface paint not to a "base" of a paint. There are lacquer, alkyd resin, or poly-vinyl acrylic or latex vehicle based enamel paints...
"There is a school of thought that says that it causes the paint to "cure" or polymerize, which may well be true. Never ever bake plastic or any metal that might melt (like some weights) and do not bake acrylic paints. Baking an enamel paint job will allow you to move on to the next application more rapidly.
I am not sure why you wouldn't bake an acrylic/latex or poly vinyl acrylic paint as it would help the surface dry harder and as the late owner (his name escapes me Greg K??? help me out here Dennis) of ACUFLEX paint recommended you do exactly that because he reminded me that, "acrylics are like Spandex and shrink to fit..." He was right his paint went on what seems to be "goby" and once the surface was heated it shrunk incredibly tight to the surface. Wonderful stuff.
"Pierre is correct. When I have baked Scalecoat 1, the resulting finish is harder, glossier, and more resistant to scratching than any other paint I've used. Allowing paint to dry "until the odor is gone" works OK, but it will not give you the same durability of finish. It will scratch more easily and wear on grabs and other wear points."
Schuyler reinforces the point that baking results in a harder, glossier and more scratch resistant finish and testifies that Scalecoat 1 is the "one". No Contest here, I don't want to start a "my doges better than your dog" thread,
The point is that heating the paint to a given temperature and holding it there for a given period of time does 1.) gas off the vehicle quicker, 2) promoting faster drying, 3) and enameling the surface faster. In the case of acrylic paints it "sucks" the paint tighter to the surface. An equally hard surface can be achieved in time if allowed to simply air dry.
"Bruce offers "polymerize." Maybe. I don't know if that's what it does or not. But whatever it does, it works.