Painting a box question

[CG]

I have a steel floor bed totally disassembled. When done, the entire inside, underside, and top of the bed rails will be coated in some sort of color matched liner. Should the bed be assembled before paint and liner, or paint the shiny parts first, then assemble and line? My guess is any sort of lining definitely needs to be done after assembly because of the thickness of the liner coat.