[LateStarter]

Re-post from July 2017
"Primers I use... (FWIW)" -- LateStarter.
[With update edits].

Over the years I've used these eight different primers. There are others to consider, and I'm sure there are favorites not included here, but this is my current list of recommendations. Each has their own pro's & con's.
Some are more useful for certain circumstances, e.g.., a light coat of Tamiya Gray will quickly reveal if lettering is de-bossed above paint-level, and then removes easily.

NOTE: Except of course for wood, I always recommend stripping before priming. At the very least, the model should be washed in soap & water, rinsed with alcohol, and thoroughly dry before priming. Surgical gloves will keep fingerprints from ruining the job.

These evaluations are based only on personal observations and opinion.
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GENERAL PURPOSE PRIMERS:
KRYLON ColorMaster (Red Oxide, Alkyd):
PRO'S: Available everywhere. Comparatively inexpensive. Excellent coverage. Dries very smooth. Good adhesion. Sprays evenly. Quick drying.
CON'S: Needs continuous rattle-shaking to stay mixed. Finish-coat retention is so-so.

RUST-OLEUM (Automotive, GRAY, Alkyd):
PRO'S: Available everywhere. Comparatively inexpensive. Good coverage. Good adhesion. Superb on metal. Sprays evenly. More durable than Krylon. Fairly quick drying.*
CON'S: Finish-coat retention is so-so. *Slower drying than Krylon.
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MODEL PRIMERS:
TAMIYA (White & Light Gray):
PRO'S: Goes on very smooth. Effortless. Good finish-coat retention. Thin coats reveal the slightest imperfections for sanding. Quick drying. Good general duty "go-to" primer.
CON'S: Doesn't hide much. Not a good choice over starkly contrasting colors, e.g., yellow stripes on black. Average adhesion. 180ml can doesn't always empty completely. Not economical.
[Note]: At this writing, white was labeled "Fine", while gray was not. I found no difference between them.

BADGER STYNYLREZ:
PRO'S: Sprays like a dream. Dries very smooth. Available in 18 colors; 2 to 32oz bottles. Adheres well to all surfaces. Good finish-coat retention. Superb coverage.
CON'S: Doesn't like to be thinned. Overspray can be difficult to remove. Not readily found or well stocked on LHS shelves. Mostly available at online outlets.

HALFORDS (spray cans):
PRO'S: Extremely durable. Hides everything. Sticks like glue to plastic, resin, and metal. Good finish-coat retention.
CON'S: Finicky. Needs to spray cool, to avoid orange peel and grit (a warm can may dry gritty). Rattle can must be shot inverted after each use. Not economical. Not generally found on LHS shelves. British product. Available on eBay.
[Note]: also available in lid-cans.

VALLEJO:
PRO'S: Sprays and brushes equally well. Dries smooth. Several colors. Shrinks nicely around small parts. Reveals minute surface flaws for sanding. Good finish-coat retention. Excellent coverage.
CON'S: Tends to not feather well when sanded. Solved with an application of Future floor wax before sanding (Look for YouTube video).

* TESTORS MODEL MASTER ENAMEL:
PRO'S: Best coverage (plastic or metal). Excellent airbrush choice. Available in rattle cans or bottles. Best primer for plastics. Thins well, or just pour-and-spray. Excellent adhesion. Excellent finish-coat retention. Dries opaque and smooth as silk.
CON'S: Very long drying time... may remain tacky for several days.

* TESTORS MODEL MASTER ACRYLIC:
PRO'S: Excellent coverage. Excellent hand-brush choice (e.g., small parts). Good Finish-coat retention. Thins easily. Dries opaque and fairly quickly.
CON'S: Average adhesion.

* NOTE: In my opinion, ModelMaster acrylic is best for brush use, and dries fairly fast. ModelMaster enamel however, (my favorite) sprays superbly, but takes forever to dry.
Pick your poison.

ALSO NOTE:
Within the hierarchy of paints --
Lacquers-Enamels-Acrylics... there is a basic rule of thumb:
"Enamels over lacquers; acrylics over enamels."
Any variation of this rule doesn't usually end well, especially if thinners are used.

 

[MichaelE]

I've always liked Krylon's etching primer. It used to come in a zinc chromate color, but these days I think it's grey.

 

[LateStarter]

ANOTHER USE FOR RUST-OLEUM... BRICK:
I use Rust-Oleum Automotive Red Primer on unpainted styrene brick material, e.g., JTT, Rix, etc. It's a good color rendition.
DullCote.
Then I 'wipe' a gray acrylic over it, to fill-in the mortar joints... (rub to clear brick faces).
DullCote again.
Follow with random wipes of darker gray (or black) to simulate mortar "age and mold" in some areas. Dark green also works.
DullCote again.
Carefully brush-paint random bricks darker colors (browns & black) for a truly realistic look.
DullCote again.
Lightly bottom-weather (airbrush) with black and a moss green color for an amazing aged brick look.
DullCote again.

 

[Dennis461]

Lets hear more...
Bake after priming metal, zamak, brass, steel?
How do these work on black plastic?

 

[LateStarter]

Lets hear more...
Bake after priming metal, zamak, brass, steel?
How do these work on black plastic?

To illustrate how coverage is over black, (or dark colors) here's two projects I did that started with blacks & darks, stripped, primed over, and finish-painted.

Primer is ModelMaster gray enamel... not thinned, one thorough coat.

Brass, Zamac, and steel should be no different.
I have no experience with baking after priming.

Bachmann 45 ton:

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Wagontop boxcar converted to SP/REA:

 

[wvgca]

I use primarily the Krylon ...
As there is a lack of local hobby stores, the Krylon is easy to get from Wal-Mart..