Acrylic lacquer eh? Lacquer is going away because it is not very environmentally friendly in the way it cures by evaporation. Too bad for the painter, because it is relatively easy to work with and doing spot or panel repairs is one of the things that everyone loved about working with lacquer. If you needed to repaint a panel, in your case the roof, you could sand it down, reshoot and blend the panel. The key was shooting a "step-down" test panel on a strip of sheetmetal so figure the number of coats needed to match the surrounding panels. Depending on how old your current paint is and the condition of it, you should be able to do exactly what you describe with lacquer.