After scraping all popcorn texture, please tell me step by step on how to texture it back with knockdown, does the surface needs to be primered before adding new texture? Does all the dust have to be wiped before appliying sheetrock texture? A contractor did my ceiling with a machine using sheetrock product.. and it is falling off when we paint it, we need to know what he did wrong.. please give me all the details!! I will really appreciated

Yes anything loose and all dust should be removed. Priming is always a great idea.

Please note, this is something that I do very little of.

1) you would use a dry mix of joint compound and a texture sprayer, spray it on.. Or you apply the mud with a 12 inch knife leaving a rough texture behind. By dipping the knife in water keeping it wet and then without using a lot of pressure apply.

2) Let set for around 20 minutes

3) using a about a 8 inch taping knife at a very low angle other wise you would end up removing the mud instead of knocking it down.

The whole idea here is to apply a texture of mud and to knock down the hight spots giving that flat looking look.