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The usual way to tape a typical butt drywall joint is to put .5 cm of drywall compound over the joint to create a bed of "mud", then add tape, press in to get excess "mud" out, then apply more "mud" over the top of everything.

This all-purpose drywall compound shrinks as it dries because the water evaporates out of it. Does this evaporative shrinking introduce voids under the tape?

This seems especially to be the case for factory edges because they're so significantly tapered that a good deal of drywall compound is required to fill the space. This means there will be a good deal of shrinking if, for instance, drywall compound loses 15% of its volume to evaporation (just an off-the-cuff guess). Tighter joints (cut so that they fit very tightly) won't suffer the same amount of shrinkage (by volume) though there are other problems with not having a good amount of drywall compound in the joint.

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    You can also use mesh tape in place of paper tape. Some prefer it, some dislike it, but it does avoid the problem of voids.
    – MickeyfAgain_BeforeExitOfSO
    Feb 10, 2020 at 13:39

2 Answers 2

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Under the tape, you should have no voids. If you do, you likely didn't press it into the bed of mud well enough, or you didn't have enough mud period. The mud acts like glue to the tape (properly applied).

Mud is designed for multiple coats, however. Even if the mud pulls the tape in some, a subsequent coat should fill the gap.

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    Oh, I see. It sounds like you're expecting that the mud will shrink, but a vacuum effect will prevent the shrinking from creating a void. Instead, the tape will "follow" the mud into the joint. Excellent, and thank you!
    – lmat - Reinstate Monica
    Feb 10, 2020 at 3:07
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    Also, excess pressure with the trowel when bedding in the tape can squeeze out so much mud as to create a void where there wasn't one before.
    – Greg Hill
    Feb 10, 2020 at 3:18
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    Using the wrong mud, or mud not mixed to the right consistency/moisture level can cause the tape to not adhere properly. So, you might not have a "void", it could just be the tape not sticking.
    – JPhi1618
    Feb 10, 2020 at 15:24
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Depends what you mean by "void"

Will it create pockets visible to the naked eye? No. Not unless you created the pocket by not working the mud in sufficiently.

Will the evacuated water leave porosity at the microscopic level? Oh yes. Yes it will. Will paint react to those microscopic voids, giving a different surface sheen? You bet it will. That's what primer is all about. Primer is designed to equalize the surface so all parts are comparable in base color, reactivity, porosity, and sheen. Precisely to stop things like that from "printing through" the paint and giving a poor result. This is the weak spot in "paint that's its own primer" - primer is a hard job, and something made to be paint can't do both well.

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