Pinholes in Gesso Re-Visited!

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Woodworks by John

Woodworks by John

CGF II, Certified Grumble Framer Level 2
Joined
Jul 4, 2000
Posts
342
Loc
North Las Vegas, NV
Business
Retired, work from home shop
Quite a while back I had this same topic and I really hate to bring it up again but --------- I'm perplexed! I'm doing another frame and have the same problem but here's the rub. The frame had a wash of rabbit skin glue (1 part RSG to 10 parts distilled water by weight) and my sample pieces did not. As soon as I brushed the gesso onto the frame the pinholes appeared but they didn't on the sample pieces. The pinholes do not go away, even after 5-6 coats of gesso.
Question is, what am I doing wrong with the initial wash? When brushing the gesso on it seems to stick, should I sand that initial wash coat? Is my ratio wrong? Am I over-brushing it? Is that initial coat really necessary? I found from one source that they added a small amount of whiting to the wash coat,would that help? At this point there's no doubt that the initial wash coat is the culprit.
Any help, advice, etc. will be much appreciated so I thank you in advance.
 
Do you think you may have let that base coat dry out too much before applying gesso?
 
Well .........

My thoughts are that since the gesso went on fine when applied to the samples excess shaking or stirring isn't the culprit. As far as the initial wash coat, that was done the day before. Could that make the difference? That crossed my mind after doing the first leg so I sprayed a bit of water on the next leg but that didn't help. I would also have thought that after applying 5 coats the pinholes would fill but not so.
How important is that wash coat? Could I seal the wood with shellac?
 
John,

My impression is that there was a heat issue, too cool, or the initial coat was not penetrating into the wood, but was drying on the surface, or the initial glue solution needed a gel suppressing agent, i.e., a tablespoon of salt to about 2/3 of a quart of solution. Once there are pinholes, well, you have pinholes.:cry:

The first coat of "whatever" needs to penetrate, not lie on the surface! As to bubbles in the solution, I, with great vigor, brush my first coat into the wood, and just as vigorously, lick it off, with the brush, so it is a penetrating wetness, not a surface coating.
 
A Quickie Test

"or the initial coat was not penetrating into the wood" Bron, what you say makes sense so I mixed up a quick solution of RSG, applied it to a piece of basswood and then soon as it felt dry (30 minutes or so) worked in a coat of gesso with no stinking pinholes! The wash I used was 1 part RSG to 10 parts water by weight, is that too gluey so it didn't penetrate? I ask though, do you need that wash coat?, that seems to be my problem.
 
I usually do not get pinholes (and on the occasions I do, I never seem to notice what I did differently that time). You provide good detail on what you did, which should help with diagnosis, but my guess, and it is only a guess, is that the low humidity of where you are may a factor.


My general procedure is: My glue size (what you are calling wash) also has an amount of whiting in it; apply with stiff bristle brush, generously, but not scrubbed. I have left in this state for days/weeks before gesso, no ill effects.

My first gesso coat is applied with a stiff bristle brush, and applied with some force (btw, I saw a demo from a French gesso recarver, who was really aggressive in scrubbing in that first coat of gesso).

Subsequent coats brushed on (or airbrushed).


Joseph
 
I use around 40 grams of glue to 20 ounces of water, varied if the resulting gesso is either too hard or too soft. I've done first coats with and without whiting; doesn't seem to matter; but either way I scrub it in and off, so there is no surface coating. The first coat acts as a primer, and because it's in the wood you have better ultimate adhesion between the wood and gesso, as well as solid coverage; thus allowing subsequent coats to be sprayed. I've done some frames, where for reasons of economy, where the gesso is only sprayed; I don't think they will last as long, but sometimes price is an issue, that outweighs longevity.

Joseph, pinholes happen sometimes.:smiley:
 
What I'm finding out!

Okay, without a doubt it's a problem with the first wash/initial coat of gesso. Now that I continue to analyze things the cause of the pinholes is that the gesso is resisting adhesion to that initial coat. I've tried several experiments including using cut shellac as an initial coat. I sanded it lightly with 400 grit and then "scrubbed" in the first gesso coat with very little of the "resisting" I'd noticed before. Maybe putting a bit of whiting in that coat would be good or ------ just forego that coat entirely and go directly to gesso. Seems that what I'm reading is that the frame is completely covered (front,back,sides, etc.) with the thinned down wash coat to equalize the effect of humidity down the road. Much like in furniture you should finish the top and bottom of a table top for the same reason.
I'm leaning towards brushing on a coat of shellac to the back and rabbet of the frame and then applying the 5-6 coats of gesso. I'm working now in what used to be the kids bathroom (repurposed as a gilding room) with a humidifier so humidity level is around 40%. I think this will save a lot of hair pulling and gnashing of teeth!
As I continue to learn, or try to, about the gilding process it seems that the rabbit skin or gelatin glue mix can stay pretty constant at 10% and the ratio of that to whiting and distilled water is what will determine whether you have a hard or soft gesso. Then for clay the glue percentage seems to increase to maybe a 15% ratio.
All things said, I can't tell you enough how I appreciate you on the Grumble helping me out on these things. Las Vegas is no place to pursue these types of things -- bangles, beads, glitz, and #%#@$, but not too much of the other forms of culture!
 
John,

I really believe, the first coat, thinned, whatever, made up of whatever, is the most important; and the most important part is "licking it off", no surface coating. Some "experts" advocate 24 hours of drying; I don't have an opinion, other than that 8-12 seems fine. Investigate urea, salt, large mammal urine, as gel suppressants; the coatings may be drying too fast to "lick off" the pinholes.

Gel suppressants retard the gelling of Jello, basically what we're using, and once there is gelling, if there are pinholes, they be permanent.
 
Ah Ha -- now I get it!

I've seen both you and Baer make reference to using urine in the gesso which was always followed by full moon, incantations, or something else so thought you guys were just making fun. The salt as a gel suppressant -- I'm catching on. It does seem that the initial coat just lays on the surface without penetrating at all. Okay, I'm ready to take another frame on. Now, work on my leaf laying techniques and toning and may have some measure of success before I hit the 90's!
 
John,

Baer and I both were kidding a little, because there are some old, guild, type recipes that include under an old oak tree, under the light of the full moon stuff. When I tell people that rabbit skin glue probably isn't from bunnies, I get some blank responses. Urea, which is a component of large mammal urine, and salt, are gel suppressants, keeping your solution of questionable bunnie skins liquid. It may help in a hot environs.
 
PASS THE SALT!

Okay, next frame I'll throw a bit of salt in my initial wash, without iodine right?
 
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