So, I have had various failures using spray primers, typically from over/under spraying and I have been looking for a paint on primer of some sort. So I ran across this article Priming with gesso. So I went down to my local art supply store and bought a bottle of the black gesso. You can also get white and tint it to any color you want. So far I have primed about 20 models and learned a little bit.
1. you don't have to dilute it 2. you should not just glop it on like the article says because you can get air bubbles in the gesso which can cause little pits when it dries as well as pooling at the bottom of arms, etc. 3. Use a good brush, I tried a cheap brush and it left a lot of bubbles. 4. most if not all of the bubbles go away. 5. you will never use that brush for anything else ever again.
I really like using it, it takes the guess work of using spray primers(humidity/space/coverage) out of priming. I would not use it as a base coat though. So far I have applied some paint over these gessoed models and the coverage is just like painting over any other primer. You can also fix any areas that you might have missed/under covered. Over coverage is almost impossible since you are literally painting the stuff on. However you can't go and prime 20+ models in a couple of minutes, but for me, unless I am doing a bunch of similar/same models that is not really a problem. I tend to work on 3-4 at a time.
If you are looking for an example of what I call bubbles/pits, go to the link and scroll down the the pictures of the Urban Mammoth Junkers Convict Legionary where the black gesso was "glopped" on. Look at the detail picture of the shield, if you look at the "elbow" of the bird you will see those little pits I was talking about. Remember that this is not completely dry yet so the pits will probably shrink, but not disappear all together.