Typically a surface treatment is used prior to paint - liquid or powder- to seal the surface and promote adhesion.

In general, custom extrusions will be more costly than profiles from an extruders "standards" catalogue, due to tooling costs, production quantities, etc. However, extrusion tooling is relatively inexpensive, so the difference is often not that great and when the ability to design-in specific desired features is considered, a custom part may be less than a machined standard.