We sanded down old texture on ceiling. Then skim coated, and sanded again. After that was dry, put up knock down to ceiling. When it dried, we have little holes all over. What can we do to fix it?
The miniscule openings you notice are from air trapped in the joint compound (knock down). It occurs when the compound is stirred or mixed rapidly or with a mixing paddle meant for a different material. Bringing the paddle blades out of the compound when mixing will add air.
It may also be occurring if the surface has been previously painted or sealed. If the air in the compound can't penetrate the sealed drywall surface it has no other direction to go, but up through the compound.
Some people claim that adding small amount of dishwashing soap decreases the air pockets by decreasing the surface tension, but I can't vouche for this .
Mixing slowly will decrease the air entrained compound. The simplest way to remove them is through sanding and then applying a lightweight compound over the holes.
If you noticed little air bubbles as you were applying it then that's what became the pock marks. This usually means you didn't add water or enough water when mixing or re-mixing the bucket.
You can dry sanding it again of course, but nobody likes that mess. I'd go with wet sanding & see if that smoothes it out enough for roller painting dimples to disappear them.
Wet sanding is nothing but wet (shy of dripping, but not wrung-out) sponging. Smaller double faced kitchen sponges would probably be best for this situation. Yellow synthetic on one side & scrubby on the other, you choose which works best.
It's frequent wringing out of the sponge but there's no dust & usually you can smooth or blend these right out. Be careful of your edges so you don't dull or round them too much, unless you find that you like it.