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I'm using 20 minute setting compound to cover a large rectangular patch. I mixed 1 lb of powder with about 1.5 cups of water, and am using a 12" knife. This picture:

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shows the the result I'm getting when spreading the compound on the wall. The ~1/8" holes (green outline) from some internet research seem to be air bubbles (I guess I'm supposed to whip/mix the mixture better to prevent those?), but what's the deal with the crazy jagged edges (red outline)?

I get those same types of very large (~2") holes in the center of the area when I go back over the area with the knife as well, so I keep going and going pressing harder and harder and eventually the big holes go away... but I'm back to having no compound on the wall haha.

How do I make this compound spread more smoothly?

  • Practice ....... more practice .......... – brhans Aug 13 at 21:39
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First, rookies should not use rapid setting compounds. Next it looks way two dry, if it is hot a quick set will not provide you enough time as it will set even faster, always use cold water never warm or hot. Fast set is also harder to sand, I would suggest sanding that down and using ether joint compound or topping mud. I can’t tell if you are trying to texture the wall or repair a patch topping mud is the softest and easiest to sand and or provide a very nice feathered edge joint compound is close but not quite as smooth and just a bit tougher to sand. Once you get good with regular mud try some 60 minute if you must, as @brhans said practice but don’t start with the most difficult type unless you like sanding. How much dried in your tray and on your knife? Practice, practice, practice. If it doesn’t look good with regular mud you scrape it off I will remix in the tray and possibly add more cold water.

  • Hm, I did add a little bit more water at one point, but then it was so thin that it was dripping all over. So I added more powder and got back to where I started. Also, to do that small job should take well under 20 minutes, so what's the concern with the "rapid setting"? – David Doria Aug 13 at 22:10
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    So how long will it take you to fix it and finish it? Probably more that 20 minutes. – JACK Aug 13 at 22:20
  • Haha na, I scraped it all off before it dried. I'll try with the pre-mixed all purpose compound thinned out a bit tonight and see if that fixes it. – David Doria Aug 13 at 23:41
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    I don't think you have to thin out the pre-mixed stuff. I never have and have gotten very satisfactory results over the years. One tip: Don't over-work the stuff. Smooth it out once, as best you can, maybe touch it one more time, but don't keep going over it again and again. That rarely makes it better and almost always just makes you mad. – Greg Nickoloff Aug 13 at 23:55
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    I've been taping since the early 70's and never use anything faster than 90; the working time is just too short, to do anything meaningful. And on that note; the working time of any setting type, does not change with the amount of water... so if you make it too runny and have to wait for it to thicken up or add more powder, your working time started when you first added water, and now you have even less time to work and a potentially lumpy mixture to boot. – tahwos Aug 15 at 2:00

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