Yes, you need to brush the dust off of the wall before applying more mud .... or primer. It doesn't need to be perfect but you should get 95% of it off.
Already made two different types of mistakes in drywall coating
I think I've already made two types of screw-ups coating some badly damaged drywall (construction adhesive residue). I'm using USG Beadex all-purpose premixed at this point.
I first tried applying unthinned mud with a hawk and trowel. I got greedy and didn't pull it out thin enough to see the previously applied tape, and since I have a poor touch with this, the thickness varied a lot over the surface (small wall in a bathroom). A few of my passes were almost smooth but others left some ripples. After 20 hours, a couple of spots were still wet enough to be putty-like, and I see one or two birds-foot shaped cracks (about 1/4" across). I will need to wait at least another day to be able to sand that. Do I need to deeply sand this crap out, or just wait until it's fully hardened to add another couple of much thinner coats to the low spots, then sand?
Since that clearly wasn't a smart approach, I tried thinning the mud with water until it would just sag off a vertical knife to leave a 1/2" layer, and coating another wall with a roller and 12" knife as I saw in a how-to video. I used only a 1/2" roller (not 3/4" or 1" as recommended) since I was now spooked that it would go on too thick, which was another mistake. I got a few thick blobs here and there (nowhere near as thick as the trowel mistakes above), but some areas were too thin and (since the wall isn't perfectly flat) the knife wouldn't smooth them well, leaving a stippled texture, and there wasn't enough excess to catch on the knife and fill in the shallow border around the edge where the roller doesn't put it down as well (which worked in the video). Some previously-filled nail-head dents also didn't fill out well, I guess again because they didn't get enough mud off the roller. But this did seem like a step in the right direction, so I think I'm going to try it again with a longer nap roller to apply another coat (or probably two, until I'm sure I have enough to sand everything out fairly flat and smooth). I've sanded the worst ridges I left with the knife before. Should I sand down all the stippled areas, or just coat over them?
Other than vacuuming the floor, does sanding dust need to be removed from the wall with a brush or vacuum between coats to get good adhesion? Or does the mud somehow suck up the dust off the surface as it goes on?
I first tried applying unthinned mud with a hawk and trowel. I got greedy and didn't pull it out thin enough to see the previously applied tape, and since I have a poor touch with this, the thickness varied a lot over the surface (small wall in a bathroom). A few of my passes were almost smooth but others left some ripples. After 20 hours, a couple of spots were still wet enough to be putty-like, and I see one or two birds-foot shaped cracks (about 1/4" across). I will need to wait at least another day to be able to sand that. Do I need to deeply sand this crap out, or just wait until it's fully hardened to add another couple of much thinner coats to the low spots, then sand?
Since that clearly wasn't a smart approach, I tried thinning the mud with water until it would just sag off a vertical knife to leave a 1/2" layer, and coating another wall with a roller and 12" knife as I saw in a how-to video. I used only a 1/2" roller (not 3/4" or 1" as recommended) since I was now spooked that it would go on too thick, which was another mistake. I got a few thick blobs here and there (nowhere near as thick as the trowel mistakes above), but some areas were too thin and (since the wall isn't perfectly flat) the knife wouldn't smooth them well, leaving a stippled texture, and there wasn't enough excess to catch on the knife and fill in the shallow border around the edge where the roller doesn't put it down as well (which worked in the video). Some previously-filled nail-head dents also didn't fill out well, I guess again because they didn't get enough mud off the roller. But this did seem like a step in the right direction, so I think I'm going to try it again with a longer nap roller to apply another coat (or probably two, until I'm sure I have enough to sand everything out fairly flat and smooth). I've sanded the worst ridges I left with the knife before. Should I sand down all the stippled areas, or just coat over them?
Other than vacuuming the floor, does sanding dust need to be removed from the wall with a brush or vacuum between coats to get good adhesion? Or does the mud somehow suck up the dust off the surface as it goes on?
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for tl:dr
Is it necessary to do a lot of sanding between coats of ugly mud, or just keep going with another coat until it's thick and uniform enough to do one big sanding?
Is it necessary to do a lot of sanding between coats of ugly mud, or just keep going with another coat until it's thick and uniform enough to do one big sanding?
You only sand at the end, not between coats of mud. Big ridges and bumps can usually be knocked off with the knife, if needed.
The only time I would consider sanding between coats of mud is if someone had done a really poor job of applying the previous coat of mud. I always do as s\s stated, scrape off any ridges if needed and only sand the final coat.
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