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When you prefill gaps in drywall, do you sand the excess that is on the wall outside of the gap completely off before applying tape and mud or will the new mud and tape adhere to the prefill?

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New mud will adhere to old mud. Just scrape off any excess to make the next coat smoother. I have done those two steps right after each other. Fill in the gaps, add a narrow skim coat, 3 or 4", add and smooth out tape. Wait for it to dry and then use larger knives to build up and feather out.

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I assume you are prefilling because the gap is too wide. Say we start from that if it be the case, otherwise there would be no need to prefill.

If you are prefilling due to large gap between two sheets etc, use the proper mud, i.e. quick drying mud. From 10 minutes to 1/2 hr time frame drying time depending on which mud you like to buy. These muds are of course the type you have to mix.

Keep the mud on thicker side. Do not add to much water. Just a comfortable thick consistancy depending on the gap. You have to judge what you got there.

With this set up, just fill the hole/gap only and no more. Scrap off excess, there should be no problem doing it.

However if the holes or gaps are too large, that is another story, it would need backing. If not. just realize to scrap off all the excess mud. Why? Because naturally you will be applying mud again before you apply the tape over it.

After you apply the mud and the tape over it, scrape off any excess beyond the tape gently, not as crucial as you would think. Do not press too much on the tape. If you notice the tape getting fuzzy on top, it means you are pressing the knife too hard and or too many time.

All that translates to that you applied too much mud to begin with before you applied the tape. This means do not apply too much mud in the first place. In other words the purpose of the mud was just to keep the tape in place. You do not want fill the area with mud but to only secure the tape. You can always put more mud later as things dry and so on.

At this stage you can move on with your life until the next day. Or you can apply a thin coat over the tape if you like and taper the mud out with a larger knife.

And there you have it. Easy as pie.

Take care.

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  • “Quick drying mud” is also known as ‘setting compound’ or ‘hot mud’. As noted, it comes powdered. Around here it’s “5”, “20”, “45”, and “90”, referring to set time. No need to go with something as fast as 5. Nov 6, 2022 at 9:49
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    @AloysiusDefenestrate, those times refer to working time, not setting time.
    – isherwood
    Jan 8 at 15:22

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