Along the "preparation" line of thinking, and as much a pain in the arce as it is, I agree that anything under the paint will ultimately affect the finish. (might hurt less to have a tube of 'Preparation H' handy
)
*greese
*old paint
*RTV sealants (silicone based)
*RUST
*oil (from assembly lube and/or hands)
*temperature of the engine casting (or parts)
I am sure there are many things more tings to consider but with a whole lot of effort, comes a whole lot of reward.