The following is a copy of a post on the Picture Framers Grumble, regarding pinholes in gesso:
"Pin Holes, ehh.
Baer, urinating in the gesso jar again? Humph!
Actually, large mammal urine contains urea, which is the compound used by TiteBond to make "liquid hide glue". It suppresses the gel rate. Hide glue and RSG are the same thing, just different glue strengths.
John, your recipe might be on the strong side. I use a ratio of 2 grams glue, dry, to 1 liquid oz. of water. For a 20 oz. jar of gesso, I add a tablespoon of uniodized salt ( the palm of my hand ). Salt is a gel suppressant as well. I've never used urea, as I'm too lazy to go to the drug store and ask the pharmicist if he has any. Baer fails to mention the "secret" aspects of the urine recipe; that it be done in the light of a full moon, shaded by an old oak tree, preferably in a cemetery. The gilders guild says so.
First coat needs to be as thin and "licked out" as possible. Wet sanding, polishing, rubbing, etc. will work, but the first coat is the important one. 1st. coat is always brushed, then subsequent coats can be sprayed. A splash of alcohol, in both the gilder and the gesso helps as well. Alcohol acts as a tension breaker, suppressing bubbles.
The problem with linseed oil is it needs to be emulsified in the gesso; a blender works well. I routinely let my gesso gel, then reheat for use. I used to use a hot plate from a coffee maker, as they heat to around 150 degrees, but now I just use the microwave, after spending some time with a thermometer figuring out times and power."
"Wet sanding, polishing, rubbing, etc. will work, but the first coat is the important one" above, is how to deal with pinholes after the fact. The first coat is where pinholes develop and then show through later coats.