Here is a short story on spray self-etch primers. We sell them all except the NAPA product. Lots and I do mean LOTS of info in the archive files.
NAPA 7220 is packaged by Sherwin-Williams company. Sherwin-Williams 988 is the same blend but slightly lighter in color. SW can has a special nozzle that spays a little nicer pattern. Both dry quickly, not real hard, and neither has any corrosion protection at all. I like the SW988 better. SEM also makes a self etch that is nice and works OK as well.
Dupont makes a product 4115S which is a self-etch that actually has a little corrosion protection (but not much). Also with special nozzle and nice pattern, drys quick and seems more durable to me than any of the other products. This is the one I prefer by far. Probably $10 a can but it goes a long way. Normally two light coats is plenty. Does smell stronger than any of the products above. The 4115S is a sorta olive color and they make a light gray which I think is 4114S.
None of the self etch rattle cans are intended to have any corrosion protection as they are designed to be topcoated. In general, most primers do not provide corrosion protection by themselves and depend on the topcoat to seal out the elements. (a few exceptions) All of Vans QBs come with Sherwin Williams self etch (not the same as SW988) so I doubt you are really adding any protection with the rattle can over what is already there.
All of the rattle can products can be easily wiped back off with a rag and lacquer thinner so that tells you there isn't much there.
The two part epoxy primers are the best from this standpoint and way better than any rattle can product for corrosion protection. If you want corrosion protection this is the better route but more trouble, expensive, and heavier. Spruce has seveal products including some water base stuff.
Attached is a picture of the Dupont rattle can product. FYI, the other is a door jam clearcoat product that is a high gloss clear that will really dry hard and durable over almost any topcoat. I did my panel with the Anodize Grey over the 4115S Dupont and then clearcoated with the high gloss and it looks like a high dollar auto finish. (don't have much panel showing so gloss isn't a problem.)
The SEM 39133 is not a self-etch but is made for flexible stuff like the carb air box or other fiberglass/plastic kinda stuff.
All of these products and more should be avialable at any Automotive Refinish Color store and you should get a lot of good advice as well. Look for someone that sells Dupont, Sherwin Williams Automtive, or PPG Ditzler products as these will typically be specialty suppliers and know the products well.
If you can find a store that sells Nobel or USAir (aircraft specialty paints) they have a couple of rattle can products as well.
Hope this helps and remember,.... it's just one opinion,...lots more out there.
Bill S
7a finishing