I bought a container of the Artist's Loft Gesso and did a few tests.
I tried it straight out of the container, but it was clearly too thick. It has the consistency of glue or thick latex paint. The un-thinned gesso actually cracked a little as it dried. So after playing around a bit, I settled on the following mixture:
1/2 teaspoon of gesso (about the size of a US quarter) + 10 drops of water. I added 2 drops of flat black acrylic paint to give it a little color too.
The results are somewhere between regular acrylic paint and milk. More like melted ice cream, if that helps.
In the photo above, you can see the unthinned and untinted gesso on that plastic spoon. On the right, you can see the thinned and tinted mixture.
Here are my two test figures:
Hullo little 2nd Ed Gretchin! Hold still, this won't hurt...
The model on the left was primed with Armory black spray primer. The model on the right had two coats of gesso mixture brushed on. As you can see, there is no discernible loss of detail on the model primed with gesso. Looks good to me anyway!
I was actually surprised at how fast the mixture dried. I let each coat dry for approximately 30 minutes. Based on other reports, I expected that it would take hours, but after just 30 minutes I found that it was dry to the touch. I did paint it on pretty thin, so that probably explains why it dried so fast. I might even apply a third layer, to make sure I get good coverage.
So to sum up: Spray primer is faster for sure, but the weather conditions need to be right. Not to mention that it's carcinogenic. I usually let sprayed models off-gas outside for at least 30 minutes before bringing them inside. I found the Artist's Loft gesso to have very low odor; I had put my nose right up to the container to even notice any smell. The downside of the gesso is that you have to brush it on, dab at any thick areas to make sure it dries evenly, and will probably need multiple coats with drying time in between. On the other hand, you can use it indoors year-round. Both methods have benefits and drawbacks. I'm going to try a few more models, but from what I can see gesso is a pretty good replacement for spray primer.
YMMV.
Thanks!