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Discussion Starter · #1 · (Edited)
This is my first experience with drywall, so as I have pointed out in my other threads, I have no real idea what I'm doing!

I thought I wanted to use a setting compound for my corner tapes, to make some deep fills for repairing a few damaged spots in the drywall, and to fill in a recessed area that I wanted to bring flush up to some trim after removing hardboard from the walls, as shown here:

I picked out USG Silver Set 90 instead of Durabond 90 since I figured that would be easer to sand down when I made a mistake and needed to remove some. It seemed to work well when doing the deep repairs, and starting some inside corner taping, but then I made a batch for doing the second coating over the tape that I couldn't seem to get to go on right (getting crumbly right in the corner instead of smoothing nicely).

So, I went back to re-read the instruction for mixing on the bag, which seem simple enough and didn't suggest any problem except that I didn't mix quite as long as they suggest, but then I read farther down and found "Application to porous surfaces or at less than 1/8" thickness may prevent chemical setting and bond." What the heck? Lowe's website recommends this stuff for taping, but that's going to be using it at WAY less than 1/8" thick.

I looked up a USG "submittal sheet" for this stuff
https://www.usg.com/content/dam/USG...ttingtype-joint-compounds-submittal-J1649.pdf
which doesn't say anything like that language on the bag. It says to tape with a thin edge, as you'd expect.

Can anyone make sense of this?

Nothing I've done seems to be falling apart, so I hope it set properly, but I may quit making this more complicated than it needs to be and pick up a bucket of green lid premix!
 

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I put paper tape in water so it is wet when I apply it, so it is not an absorbent porous surface. The problem is a thin layer over something that sucks the moisture out of the compound before it can dry. Put on a very thin coat of mud, put on saturated tape and trowel it out. Let it dry and continue additional coats. You will be replicating what a professional’s banjo tool does, it puts on a thin layer of mud with saturated tape.
 

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Because it can dry before it sets. It gets its strength from a chemical setting process, not from drying. If it dries too quickly it won't get strong. This isn't a problem out on the feathered edges. It's mostly talking about the tape embedding layer, which should indeed be at least 1/8". The paper itself also helps keep it damp, but if using mesh tape it's more of an issue.
 
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