Before painting a small item (think of a plastic model), I wash it in soapy water to remove oily fingerprints etc. Most commonly I use diluted dishwashing liquid. From experience, washing is important, especially for non-porous surfaces like plastic or metal and paints with poor adhesion (like acrylic).
The question is: how important is it to completely wash the soap residue off the surface for the paint quality/adhesion?
I'm not talking of anything obvious: the dried surface always looks perfectly clean. But if I don't flush it well and smear a wet finger over the surface, I might feel slight soapiness. Is it bad for the paint?
If we need to narrow it down, let's say the paint is acrylic (alcohol-based) and applied with an airbrush (the goal is even coverage without strokes).
Consider that it may be difficult to wash the soap off completely: often the piece is too fragile or too big to be flushed with running water, and some parts of it may not be water-proof. Usually I have to apply both the detergent and clean water with a soft brush.
Theoretically, dishwashing liquid is a surfactant, so if anything, it should help the paint (unless it has a specific reaction to the dye). But maybe someone has direct experience with this.
I didn't notice anything wrong myself, but I do try to wash it off well, if not perfectly. This is a tedious process. Maybe I'm overzealous...