Why Use Joint Compound For Plaster Repair?

Joint compound is my material of choice when it comes to repairing interior plaster in your New York home. Why not use real plaster, you ask?

After nearly 40 years of doing this stuff, I can say - because joint mud rocks!

I have tried to use real plaster for patching holes in plaster. It works, sort of. But you need some way to tie the old and new plaster together. Joint tape and mud is the way to do that.

It is very difficult, in my experience, to patch plaster with plaster and get the patch to blend. Joint compound is much easier to use, and you can sand it after it dries to remove minor tool marks.

The other nice thing about joint compound is that you can take your time, unless you are using hot mud (more about that later). Real plaster has a limited working time, and is just about impossible to sand. Plasterers have hired me to blend their repairs into the surrounding plaster, using joint compound.

Plasterers will complain by saying that joint compound is not as tough or hard as plaster. They are right about that. But in most cases, that is not a major concern, except perhaps in some high traffic areas, as
in public buildings.

What kind of joint compound do I use for fixing plaster?

There are two basic categories of drywall joint compound: regular mud, and quick set (hot) mud. Let's talk about the first one.

Regular joint compound usually comes premixed, in buckets or plastic lined boxes. In the "old days" when I was starting out, my boss liked to mix his own, so he bought sacks of powder. I don't see sacks of joint compound any more on the west coast (US) where I live, except for texture compound - and now you can even buy that premixed.

The basic feature of this type of mud is that it cannot harden until it has dried on the wall or ceiling. Most
US drywall people use the hand-held machine tools like the bazooka and the boxes for finishing drywall, so
this is the mud they apply to drywall seams and corners. Because this mud dries slowly, it has a tendency to shrink some, and is not very good on that account for filling holes or for thick applications.

This regular kind of compound comes as taping mud (for sticking down drywall paper tape), all-purpose mud (which can be used for any step of the finishing process) and topping mud (used only for covering
the taped joints, NOT for taping). For the homeowner, the all-purpose is the one to use. With it, you can apply tape, cover the tape, spot the screws or nails, and even use as texture. It works great for taping
and finishing plaster cracks.

But there is another distinction you need to be aware of. Regular joint compound comes as full-strength, and in a lightweight version. The full-strength version is heavier and tougher, but a little harder to sand.
What works nice for plaster repair is to use the full-strength for basic taping and first topping coat, and
use the lightweight version for the final topping, for ease of sanding.

Now we come to the quick set muds, aka hot mud. I use this a lot because it is tough, doesn't shrink after
it sets up, and can be used for filling deeper cavities. But, as with real plaster, you have a ticking clock as you use hot mud. So the trick is, if you are just learning and need more time, mix small batches at a time and use the longer timed mud, like one hour or ninety minutes.

Hot mud allows me to do multiple operations in a row, without having to wait overnight for each application to dry. This makes me more efficient and shortens the time it takes to complete the job. Being that the hot mud is harder than regular mud, I usually do the final top coat with regular lightweight mud.

 

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