forgive my ignorance - but its the path to knowledge. Can you define for me the difference of "sealer coat" and "semi gloss urethane". I get that the urethane provides a protective coating but is not scratch resistant as opposed to the sealer coat which would bond to the layer of paint underneath and provide some resistant to scratches. Or do I have that backwards and the urethane is protective armour for the paint?
In automotive painting, the layers most commonly look like this:
Primer -> 2k primer / filler / glaze (to fill/level imperfections) -> Sealer -> Top coat
There's no need to block sand parts like this, so you typically skip the "2k primer/ filler / glaze".
That leaves
Primer -> Sealer -> Top coat
The two common types of primer and sealer are epoxy and urethane. I prefer epoxy because it has the best adhesion and moisture/chemical resistance. Also, by thinning it with urethane reducer, it turns into a sealer. This allows one to stock a single product for both uses.
What is sealer for? In the "2k primer / filler / glaze" stage, you'll be doing a lot of sanding and the panel will be several different colors: bare metal where you sanded through, the primer color, the 2k color, and the glaze color. That's not good. Also, with exception of the primer (if epoxy type), none of those are moisture/chemical resistant. For those reasons, you need a final layer to give even color and protect them from the elements. That's what sealer is for.
Basically, a sealer (either thinned epoxy or urethane type) is a very thin and smooth coating that seals any sand throughs and shouldn't need much (if any) finish sanding, and leaves an even color across the surface so the top coat appears consistent.
So back to "Primer -> Sealer -> Top coat". I didn't need to seal the primer layer because it was epoxy based and the sealer is literally the same product thinned. I used it for a different purpose. I didn't need to, but I chose to wet sand the primer, so this probably left sand throughs to bare metal, and possibly some light sand scratch. I also don't remember how many days it had been since I sprayed the previous primer layer. Epoxy primer typically gives you a few days to a few weeks to spray the next coat with good chemical adhesion. After that it's cured too far and becomes too hard and inert for anything to adhere well. All together, I probably sprayed the sealer coat to seal any sand throughs, to fill minor sand scratch, and to reset the clock on the adhesion time window so that the top coat would adhere well.
For the top coat, that's typically either a base coat (color) and urethane clear coat (most common for car bodies), or a urethane single stage paint that combines both into one product. There are pros and cons to both. Few people are going to go to the trouble of base/clear for engine parts, brackets, etc though, so single stage is easier.
Honestly, the part was probably fine in just epoxy primer since it wouldn't be exposed to UV under the bonnet, but I already had the black single stage and it does look a bit nicer, so I decided to go the extra mile.
This is generally what I recommend:
Black parts like brackets:
Prep metal -> Epoxy primer
Black parts exposed to UV
Prep metal -> Epoxy primer -> Any clear coat or black top coat, even spray can types.
Parts that you want to look really nice:
Prep metal -> Epoxy Primer -> 2k primer / filler / glaze (to fill/level imperfections) -> Sealer -> Top coat
Or you could skip all that and have everything powder coated.
