This is a rough explanation and purely my opinions based on my experience. It is also a canned answer that is similar to other posts I have done. This question comes up a lot! Hope it helps.
Always try to stay within a system. I prefer DuPont, Standox or Spies Hecker but I am going to make this generic. Read the tech sheets on the products you are looking to use. They will tell you what works with what and how to mix and spray the products. Read ALL of the tech sheet! Always do a spray out to check color. And take a trial run before painting the real deal. When all else fails ask the distributor, call a tech line for the paint company or try to contact the local rep.
Etch or wash primers – Go direct to bare metal and assist in adhesion but are mostly there to serve as a conversion coating to prevent a rock chip from getting out of control. Do not put over filler. Do not put under filler.
Epoxies – Can go direct to metal and work well by themselves. There is debate as to weather it is best to put over an etch or not, that’s personal preference and experience. I have done both ways with success. Can go over body filler and can go under body filler once the epoxy is properly prepped. Epoxies can also be used as a sealer. That said you can put urethane primers over them or under them and color can also be put directly on them as well. Watch your dry times epoxies are slow. Epoxies also have little filling capability but with a couple of coats they can prevent corrosion quite well.
Urethane Primers – Must have a etch or epoxy under them except for small (quarter or style line) cut troughs. Some companies recommend a sealer some don’t so follow manufacturer recommendations.
They can go directly over body filler (180 is the coarsest grit scratch I am aware of) but body filler should not be put over them (some glazing putties are okay). They will not prevent moisture from saturating through to metal so don’t put a car outside and allow it to soak. Wet sanding is okay. Also they are not body filler!!! 3 coats let it dry and sand it and reprime if necessary and watch flash times. A good filler primer builds 4-6 mils a crisp dollar bill is 2 mils thick and new OEM paint jobs are an average of 4-6 total. That’s all the more primer will fill.
*DO NOT USE LAQUER PRIMERS!!!
Sealers – Some companies do not require. They will fill at most a 320 grit scratch. They are meant to go over primers and cover very very small cut throughs (to metal or filler) and help with the porosity of primers for better color hold out. It also makes for a uniform color to go over resulting in faster hiding and better color uniformity. Some companies use colored sealers and some use various shades of grey and both work.
From here on all you have to do is pick a color and decide if you want to spray basecoat clear coat or single stage!
Single Stage - There are many urethane single stage and enamel paints available and they will give a similar appearance to OEM just shinier! I would only single stage solid colors not metallic’s. If your spraying a urethane and really want to have some fun cocktail the single stage color and some clear on the last coat looks awesome!
Basecoat/Clearcoat – This is how most new cars are painted. Must go over primer, epoxy or sealer.
Basecoat - Since you only have to worry about making the base uniform it is easier to control the color especially in metallics.
Clearcoat - Then it is time for clear and there are loads of different clears out there. Clears can be very environment and skill level dependant so ask questions. Higher solids clears will look deeper and have better DOI (distinctiveness of image) but may be more challenging to spray.
Color - Depending on plant,enviroment and spray equipment colors varied. Even today there is a great deal of variation on color. Look at all the variants for GM's 382E Pewter! When spraying; the color you go over, number of coats, type of gun, fluid nozzle on gun, distance, air pressure, reduction ratio, reducer selection, type of paint (laquer, enamel, urethane, basecoat clearcoat or single stage)and paint manufacturer can all vary a color. If thats not enough golds and pewters can be the most difficult. A spectraphotometer (color camera) can help but it can still get weird. So to get started get some paint and do a spray out card. ALWAYS START WITH A SPRAYOUT. and verify it to the car near the area you will be painting. If it doesn"t match take in a polished part of the car at least 3"x3" (piece must be flat) and have a reading done then do another spray out. Some distributors even offer to tint colors for a fee. If you want what was "correct" for that car you should match it to an unmolested area and check around the car in case of repaint. Also make sure if the paint has sat that it gets shaken thoroughly as pigments settle.
As for what’s best that’s like asking what’s your favorite color. However that does not mean just go out and wing it. There are pros and cons to whatever you choose. Also some processes are better suited for certain projects or portions of the car. You could use a few different products and processes for one car. Products come and go and change so processes and recommendations can change at the drop of a hat. There is a tool for every job and paint can be the same. It takes years for people to get good at painting so keep that in mind as well.
Also make sure to wear your Personal Protective Equipment!
For FM3 I like single stage w/clear cocktail on last coat.
Good luck.
Geno
