I agree with Matt re photo-etch and resin parts - both must be cleaned, and then primed, and then can be painted as normal.
Resin parts are normally, but not always, conversion parts. For instance, a different shape of wing or engine cowling, to change the Mark of an aircraft, example Ju88A-4 to Ju88A-1.
However, they can also be, for instance, cockpit side walls, with more detail than that provided in a particular kit, and possibly to correct errors. Others can be better detailed individual items, which will also be more accurate, such as wheels, exhausts etc. This is due to the limitations of the plastic injection moulding process again, but in this instance, due to cost restraints versus the intended retail price of a kit. For example, plastic injection moulded exhausts can be made with hollow outlets, but this one item could double the cost of the moulding tool which might contain 50 other parts which don't require this part of the process.
Yes, some seats can be and are made from resin parts, but they can also be made in photo-etch, depending on the shape of the original, for even better finesse, but these will require bending and assembly.