My drywall guy refuses to do knock down texture unless it is primed first. For him it is an insurance policy. He has leaned the hard way just like you. It can be done without but it is much safer to have it all primed that way it dries equally and in a more manageable time frame. He waters down the primer quite a bit and rolls with 18" nap. Doesn't take long at all.
This can happen too if your using a wide knockdown knife AND the joints are not floated out well enough, even if they are not visually flashing, the knife will take off more mud on the higher areas than the rest of the wall.
Might explain why the joint areas are turning out different.
I learnt this the hardway, mudbox ran just a bit too full and I was using a 14 inch knife, hardly noticeable differences around the joints, but enough that I chose fix it by spraying again, as thick as the first time all over again and knocking it down with a smaller knife, and it looked really good after. In fact the builder commented on the "fullness" and "dimension" of the texture and asked that I do all his houses exactly like that....:sad:
Primer before doing a spray knockdown is best. At least prime the mud joints. You can get away without primer if when you wet the bands and butt joints after you have completed spraying a room with another light coat of spray. Then check drying, and "pre knockdown" the mud joints lightly before knocking down the whole wall or ceiling. Another thing to remember is to knock everything down in the same direction. The texture blobs elongate in the direction of the blade. I knock walls down in a straight vertical direction. Then I return and knock them down again in a straight horizontal direction. If done properly the blobs are nice and round and not elongated in one axis. Never do swirly or curving knockdown blade patterns unless you are sure that is what you want. Ahhh - spray textures. Those were the good ole days. Lately here in Western Colorado everything this last year has gone level 5 with lots of Fry Reglit details. So labor intensive it's hard to make any money. Level 5 or some kind of plaster finish is all the vogue this year.
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