Painting the parts before or after assembly depends on which parts you're working on. In some cases, usually small parts, it may be better to paint them before assembly. Other parts, like an engine may be better to be painted as an assembled sub assembly. Large parts, like a fuselage or car body are better painted as a unit. This means taping parts such as fuselage halves together and painting them at the same time so all of the parts will have an even paint job.