When it comes to painting any sort of plastic that you don't know the adhesion properties of, or where you can not properly prep the surface, what you want is an adhesion promoter. I prefer the sort that is applied separately from primer and dries clear, not the all in one sort.
I have never used the low cost options such as Dupli-Color, so I don't know how they compare to SEM or 3M's product, but I expect it is good enough for the job. The name brands cost around $25 for a spray can, but one can lasted months in an auto body shop, it will probably last a lifetime spraying an occasional engine shell.
After applying the adhesion promoter, if you choose to use a primer, I recommend you use one specifically intended for things like plastic models. Automotive primer is fantastic for its intended purpose, but it will hide details on models. Automotive primer is designed to bridge small scratches and fill uneven surfaces, then to be sanded flat, which is not desirable if you want to preserve fine details.
One last note, Prep work is important. make sure the parts to be painted are absolutely clean. You can buy 'wax and grease' removers at auto parts/auto paint stores, but for something like a couple of small engine parts, cleaning with soap and water, rinsing VERY throughly, then letting dry completely will probably be enough to insure a good bond. Once the parts are cleaned, they should not be handled with bare hands... wear gloves.
And yes, I understand that there is a difference in the process required to get satisfactory results for a model train engine versus a high-end, show quality, automotive finish, but taking your time and doing it right the first time will make for a better, longer lasting, finish on anything.
JGL
