I’ve also had good success with VMS metal prep. Never had any paint chipping issues with it, at least from a level of handling that wouldn’t chip paint over plastic.
That said seems to be a bit trickier to use on metal gun barrels as gravity causes it to run a bit before drying resulting it run marks. A few light coats rather than one thicker one seems to get round this issue.
I use AK’s Brass Photoetch Burnishing. Soak it for a few minutes until it turns black let it dry and wipe it with a soft cloth to remove the black.
PE is different than plastic. Lacquer paint has solvents that bond with the plastic. Acrylic paints don’t bond with the plastic and is easier to come off. People like to use primers to have a better bond to the plastic AND to find and cover minor imperfections. Honestly I use primer more for discovering imperfections I overlooked. PE is like working with acrylics in that the hobby paint does not bond with the metal like acrylics do not bond with the plastic. You can use something like an automotive primer that is designed to work with metal. However Gino’s solution of using a fine grit sand paper to increase the holding ability of the paint to the metal is a sound idea. Automotive shops do that also before painting. You probably do not need primer on PE to look for imperfections so sanding the surface of the PE is probably all you need. How much wear and tear do you expect your PE to go through?
I gently rub the whole surface with steel wool before cutting any parts of, then clean it with acetone and prime it with a grey metal primer.