Hi Hv5097,
Thank you for reviewing Behr ULTRA Ceiling Paint. I'm very sorry to hear about the flashing issue experienced, as it certainly isn't what we want our customers to go through.
Unsealed knockdown tends to be very porous and should ideally be sealed before painting to achieve a uniform finish. If the knockdown isn't sealed first, it can absorb paint at different rates creating an uneven look.
I encourage you to contact Behr directly for further assistance with this matter. We'd love to help!
Kindest regards,
Doris