# plaster over drywall: bonding agents



## stick\shift (Mar 23, 2015)

Geez, I would use much thicker drywall than that. At this point, I would add another layer to what you have.


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## JourneymanBrian (Apr 1, 2015)

just prime it to reduce absorption, then you can plaster.


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## ToolSeeker (Sep 19, 2012)

Use at least a 1/2" piece the less you have to fill the better.


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## diy888 (Jan 18, 2009)

I spent a while searching the web and found another product called Plaster Weld.

I'm going to apply two coats, and the manufacturer says the scratch coat should be at least 3/8" thick, so I'm ok with using the 3/8" drywall.

In any case, it's already up, and I spent a couple of hours installing it, since the area is in a stairwell with a sloped ceiling, so it took some careful cutting.


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## Nestor_Kelebay (Jun 17, 2008)

DIY888:

What you have is something called "Gyproc Lath Plaster". The Gyproc is the 4 foot by 12 inch (IIRC) panels nailed to your studs that are constructed very much like drywall. Gyproc Lath is the missing link in the evolution of wood lath into drywall.

I can see that you're wanting to repair this hole using the same materials and techniques that plasterers used 70+ years ago. What you need to understand is that it took those guys years of practice to be able to spread plaster smoothly, which is something that you're not going to master with a bit of practice. I admire you're fortitude in trying to learn the art of plastering, but it's something I myself tried doing years ago and found that I simply couldn't get as good results using real lime putty based plaster as I could using modern plasters.

75 years ago, a plasterer would have mixed Quicklime with water to make Hydrated Lime, which is an extremely exothermic reaction and can be dangerous if you're not prepared to handle the amount of heat that's going to be produced. He would then mix that Hydrated Lime with Plaster of Paris to make what was called "gauging coat plaster". But both Quick Lime and Plaster of Paris were expensive, so no one covered the wood or Gyproc lath with gauging plaster. Instead, they would add sand and in some cases horse hair to the gauging plaster to make "base coat plaster". Base coat plaster is the coarse layer between the Gyproc Lath and the top white gauging coat of plaster. It took a lot of experience to mixe the base coat plaster so that it wasn't too heavy with sand that it would literally fall off the lath before it stiffened up. At the same time, not adding enough sand would result in a much more expensive plastering job which could result in the contractor losing money on the job.

After the base coat plaster, the top white coat (the gauging coat) consisted of Hydrated lime mixed with Plaster of Paris.

Noawadays there are base coat plasters that make applying the base coat plaster a no-brainer. Domtar's Perlite Admix Hardwall and USG's "Structolite" both contain powdered glues in them to make them stick well to any kind of lath. Also, the both contain light aggregate like perlite to bulk them up without adding much weight.

So, If I wanted to repair this wall as best I could while still benefitting from the miracle of modern chemistry, I would apply any company's base coat plaster to your drywall repair. The basecoat plaster will stick, but if you want to enhance the bond, paint the surface of the drywall with white wood glue dilluted with water to a paintable consistancy Allow that glue to dry if you want; the moisture from the base coat plaster will reactivate it so that it bonds the base coat plaster to the drywall patch.

Don't fill the basecoat plaster up flush with the surrounding wall. You want to leave some space for our top coat.

Use a drywall joint compound (one that's meant for the final coat when drywalling, (AKA "Finishing" compound) or an "All Purpose" joint compound rather then a "Taping" joint compound. The "Regular" or Taping joint compounds have more powdered glue added to the bag to make the joint compound stick very much better, but also dry very much harder. You don't want to buy a joint compound that's going to be a hard slog to sand smooth.

I think all this other stuff is things people want to sell you that you really don't need and will probably never use up. It'll be stuff that'll collect in your basement until you throw it out.


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## diy888 (Jan 18, 2009)

Thanks for the tip about the diluted wood glue and the sandable joint compounds, Nestor.

Last year I did some plastering with StructoLite over bare metal lath (no gypsum board) over a much larger area, including a section of ceiling that was about 6' x 8'. I did it in three coats. No sand or horsehair, just less water on the first coat and more water added for the skim coat. Plastering the ceiling was back-breaking work. 

My technique spreading the plaster on the wall got to be halfway decent. But if you look closely at the line where the wall meets the ceiling, you would think the plasterer had had one too many beers at lunch.


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## ront02769 (Nov 28, 2008)

You are at 3/8 and need to get 7/8? Simplest would be to just hang 3/8" BLUE board and then tape and plaster skim coat it. A lot of new houses in my area go that route as it is faster, produces a better finished product and is relatively comparable in cost.


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## restored (11 mo ago)

stick\shift said:


> Geez, I would use much thicker drywall than that. At this point, I would add another layer to what you have.


Honestly the man asked you a very DIRECT question. If he needs to get out to 3/4 or 7/8" it sounds like he is going over something that is existing. Doing this for about 45 years , I'm thinking just maybe he doesn't want to remove all the existing plaster on the wood laths of which based on my experience can vary from a whole 1/8" thick to the basic 1/2" plus. If the surface being attached to is solid, I have used 3/8 numerous times without issue. There are various reasons for not using more than 3/8. The biggest is most plaster jobs from years past butted the finish trim, door window, baseboards. Typically that left approx. a 1/4" reveal. Once he jumps up in size as you suggest, a reveal may be non existent and then the cost of the project becomes huge. In both material and labor cost. Each job has different variables and being consistent with economics always plays a factor. To help answer his question, typically blue board accepts plaster better and is what we use usually. It is difficult to find in 3/8 but I have used it. If your spanning normal 14-1/2" stud spacing 3/8" is pushing the limit and in most cases not enough. However, if your adding a 3/16" of veneer plaster this will add as much strength as you would have in a typical 1/2" drywall job. I feel your pain and I think the biggest question is the amount of varying answers when it comes to bonding. There seems to be only one product being pushed on the web. In some forums white wood glue and warm water mixed is discussed as a decent bonding agent. The difference between the blue board and drywall is the paper bonded to the rock itself. I dislike paying 60 a gallon for the name brand bonding agent, and am always a bit apprehensive making my own when dealing with old plaster and new drywall where they meet and the question of bonding to both without issue. I don't mean to be a pain, your question was specific, and it appeared you had already been researching your options with the various issues that are a part of every job. Consider when paneling was the thing back in the 60's most people applied it to 3/8" drywall. This keep th trimming process in a reasonable ballpark for extension jambs and similar industry standard issues.


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