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:
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?
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.