Metal: Primer or No Primer
To prime or not to prime is a complicated subject. This section has to do with painting metal.Being a model railroader myself, one of our biggest challenges is painting brass models.  Brass or not, many of the challenges of painting brass are applicable to other metals.  Simply stated, the problem with painting brass is the oxidation that accumulates on the surface immediately after any surface preparation.  The more time for oxidation, the less chance you have for adhering your paint.  A brass model will usually have a finish color and a lacquer finish.  If you plan to remove the lacquer, a strong paint remover works well.  For early models that do not have this finish, you will often have dark areas of the model where people have touched the brass and left oils, salt, and acids which enhanced the oxidation of the brass.  We typically would just say these models were badly tarnished.  Tarnish to brass is like rust to steel.  To have a successful paint job for both, one has to remove all oxidation.  The easiest solution is sandblast.  Although this sounds difficult the equipment isn't very expensive if you already have an air compressor for painting.  One can get a simple sandblasting nozzle and gun from Harbor Freight for around $20 - https://www.harborfreight.com/21-oz-hopper-gravity-feed-spot-blaster-gun-95793.html.   I get my sand from Lowes or Home Depot for a couple of bucks.  You'll want to sieve the cheap sand because is has some larger sized particles that can clog the sandblasting nozzle.  Now that you have exposed metal, the goal is to get the surface cleaned as quickly as possible and minimize the exposure to air.  My technique is to wash the parts with window cleaner in a large pan so all the items can remain submerged until the final rinse.  If you really want a superior binding paint job, the professionals add one more rinse with a chemical preparation.  To prepare a metal surface for painting, one really needs to use a chemical prep.  Now days with the internet, one can find all different types and for different prices.  One I used in the past was a prep for aluminum which an auto body shop guy had sold me.  There were two components, a wash and a rinse.  The stuff worked great, and would even turn the brass a slightly pink color.  I wasn't sure what the stuff was, and this in the days before MSDS information.  After learning more in college, my guess, it was a weak acid and the rinse was a neutralizer.  I have seen similar things now on the internet, but they all cost money.  As a substitute to having to buy something special, a number of modelers recommended using vinegar.  I started using vinegar and I like the cost and results.  I would still get the pink tone to the brass but not to the magnitude that would appear with the commercially available chemical washes.  Again, all the parts need to stay submerged until you are ready to dry and paint.The next step is an option and depends on the size of your project.  Keep in mind the oxidation starts as soon as the brass is exposed to air.  That means oxidation is building all the time that the items are drying.  If I have a lot of parts, I use a hair dryer and dry them the best I can.  Then line them up and give them a quick coat of flat Rustoleum.  I focus on the surface that will be exposed or handled.  The coverage doesn't have to be complete, just a film to block the oxidation.  I also do this step if the parts have multiple colors and need protected until painting the final color.  There are a number of risks when spraying oil based paints from an aerosol can.  You can chase a drop of moisture out from a crack that blemishes the oil painted surface, and spraying from an aerosol can can be tricky.  You'll also want to wait a few days while the model bakes to completely cure the oil based paint.  However, if you have a well prepared surface without any oxidation, you can proceed to painting with Modelers Paint without the primer coat.  Our paint will bond extremely well to any cleaned surface.  In addition, the glues we use in the paint can be baked which will give you that same feel that the old oil based paints of Floquil and Scalecoat used to provide.  One other advantage, because Modelers Paint is water based, a slight film of moisture is not a show stopper.  I take my model from the wash pan, blow it off with the airbrush, and then use the double action feature of the airbrush to spray a rinse of 91% alcohol over the model.  The alcohol captures any large drops of moisture, and the drops are blown away with the air from the airbrush.  The process is quick and thus minimizes the time the metal is exposed to air.  Once rinsed, you can begin painting and don't need to worry about moisture.