For what purpose, drawing, rendering, mass calculations, etc?
One way to do it would be to offset a bunch of surfaces.
I would use the Offset Surface in each individual part. Or you could make a new part in the assembly called "paint" and offset faces from individual parts. You may have problems creating sections of surfaces, though. If you need to make the paint solid, you can use the Thicken tool to do that, but you'll have to be careful about where surfaces intersect or cross. The surfacing involved is not difficult, you have to know how to work withsurfaces in order to make this process work.
If you don't need it to be solid, that's probably better.
I gave this a go on a project last year to try to figure out how much paint it would take. I made a part copy of the base assembly then multi materialed it then thickened the exterior of the body, it was a bit sloppy but it gave me a plausible result. It was still off by something like 60% next to the actual in volume but I think our painter put the paint down very thick, should have been like 15mil (.015" ~ .38mm).
Okay i'm a bit late here
But here's a link from the past with more info about the topic.
I have added another small video where he's (And I think it's from Martin Bernhard) explaining how to do it the other way. Enjoy !
And it's standard SE funtionality believe it or not