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why drywall mud outgassing

donmathis's picture

I have done remodeling for 37 years & finished drywall and repaired plaster using drywall methods for 35 years. There is an unsolved problem that comes up unpredictably on painted drywall or plaster that remains unsolved and unexplained. I call them "hot spots". When the mud (I use only All Purpose) is applied there is a chemical "bubbling" resulting in scattered "pores" in a shotgun pattern. This can be minimized by priming, but not always eliminated. I called USG once and the technician diagnosed it as either outside air infiltration or air in the mud. From the locations and the experience itself I can't accept that answer. There appears to be a chemical reaction going on and I have suspected (though not tested) something like aluminum oxide from previous sanding (we sponge). We don't have this problem on new drywall.

Does anyone have the answer to this? I would appreciate it.

(post #55308, reply #42 of 73)

Thats right .


Tim Mooney

 

(post #55308, reply #43 of 73)

I see this in new construction when you coat hot mud with pre-mix. No?


Funny I never had the problem in old work.


 

(post #55308, reply #46 of 73)

Just to ask you a question in the same vein. I am having the painter and the drywall company come out to our new home of eight months to review with the building contractor our problem. Seems that the drywall mud over the seams, nail holes, and taping at the wall/ceiling connection show through the flat latex paint job. We even applied another coat of colored paint by our own sub and it still is popping through. We used this painter on our last home and his one coat paint over existing white latex came out fine with no problems. He is coming to the meeting as well. What are the possibilities for this effect? It took 4-6 months for it to start showing after our painter rolled this pale yellow/beige color on. WADR. GW

(post #55308, reply #47 of 73)

Greg , that needs to be posted under its own new post . Just respect to the poster here , as its called hijacking a thread. I know you didnt mean to do that but think about if you were waiting on your thread to be complete with answers ,  and someone knocked it off track. Your question is another discussion that we would be more than willing to discuss.


Tim Mooney 

 

(post #55308, reply #49 of 73)

I guess I just got lazy tonight. That is why I asked the original poster if he would respond, since he seemed to be an experienced drywaller who would know the answer to my question.  Answering questions within questions on the old site used to be looser after many posts already addressed the original posters question.  New threads are good, but I am to lazy tonight. GW


Edited 12/10/2002 11:14:43 PM ET by Greg Warren

(post #55308, reply #48 of 73)

Start a new thread. This is a dif. question and will complicate the answer process.

BTW the problem is not the paint. It's the mud job. Another coat of paint will only act as a temp solution.


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(post #55308, reply #51 of 73)

Sorry Greg, I don't know. I have seen Kilz spotted in show through the finish coat with a different something. We are thinking that when doing slick finish the eye must be able to see a thousanth variation in unflattering light. That is why we get the drywall to a certain point and seal it off with First Coat. If you don't it seems anything you touch it with (including paint) causes a possible shrink back. We do our touch ups and sponge whisps with a bare bulb and re prime. That brings the job up to an A.

(post #55308, reply #52 of 73)

Thanks for the reply. Sounds like First Coat was never applied. It was a production job, and I believe that  the painter only sprayed the drywall once after the texture was applied. GW

(post #55308, reply #68 of 73)

The first coat of mud has dried and thus, contains air. As the first coat absorbs moisture from the second coat, the air gets forced out. With drywall or any other pourous surface the air goes into that surface. If it is painted however, the air has to escape through the freshly applied coat, causing the bubbling.


The best and most efficient way I found to deal with this is to let it go ahead and bubble. Later after I've sanded and primed (mashing primer into the BB holes) I go around with a 6" knife and light weight spackle and fill 'em up. If the final paint will have any sort of sheen (satin, pearl, semiglos...), I go around and reprime those areas.


~ WebTrooper ~


I was wrong once and it could happen again. 

(post #55308, reply #69 of 73)

Curious, What do you mean by "air".

(post #55308, reply #71 of 73)

"The first coat of mud has dried and thus, contains air. As the first coat absorbs moisture from the second coat, the air gets forced out. With drywall or any other pourous surface the air goes into that surface. If it is painted however, the air has to escape through the freshly applied coat, causing the bubbling."


 


I said that , but I couldnt give it away!


Tim Mooney

 

(post #55308, reply #72 of 73)

I confused by this. There is no air in dried mud. If there were it would flake off. OF course this is if I'm following you anyway. When gypsum and lime are dry the they have a "ite" name. When wet, they are "ate." When they dry or cure again, they revert to "ite." This is like carbonate soda that is used as a drink mixer. Let it site long enough, and all the carbon dioxide(same with plaster and mud) is released via a chemical reaction with the "air". When released, the soda is what we usually refer to as "flat." If mud and plaster does not go "flat" it will delaminate and flake off the wall. This is one of the difficulties of working with plaster. The finishing of this process is often mechanical, and if you don't know at what time to go back at the plaster, you're sunk. Too late, or too early, doesn't matter. Better living through chemistry has made some of the available plasters today not nearly as sensitive to this as others or traditional plasters. But then again, I'm still not sure what is meant by air. Plaster and muds don't just dry by evaporation. The oxygen is pulled out of the H2O to create the carbon dioxide. In hot muds, the rate at which this happens is chemically controled and that's how they work.


Don

(post #55308, reply #73 of 73)

"I confused by this. There is no air in dried mud. If there were it would flake off. OF course this is if I'm following you anyway."


You arent following me , but thats normal for me ! LOL!


In a fill coat application running recessed joints , or joints that are high grade such as the side of a butt that is bad , we leave a lot of mud in a" fill coat" Air and water in unprimed drywall will suck in to the drywall because that is the polarized reaction until dry. You are right in saying that dry mud doesnt contain air . It happens when the mud is still wet causing bubbles to the top in cases of deep fills . Mechanical or hand operations doesnt trowel that coat where its deepest. Later in skim coats it is troweled ; thus meeting the surface. Sometimes filling new drywall causes open air pockets in the mud , but they are never trapped as you are misunderstanding me. There is quite a bit of air in a box  of USG Pluss  3. Of course adding water and mixing to a creamy state reduces the air , but in a heavy fill , I dont add much water if at all. Mechanical fill coats will have a small amount of water,  not enough to release the air as its almost straight mud that is mixed .


Anyway, I dont worry about it because I know that well thinned coats will follow , filling the small voids.


Tim Mooney