We just bought a house and I am painting it before we move in. The kitchen has nicotine stains and semi-gloss or perhaps even gloss paint on one half of the ceiling. There is a crappy line that runs down the middle of the ceiling between the cooking area and the eating area where the paint changes from beige colored gloss paint to white flat ceiling paint

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I am cleaning the whole house with TSP to get the nicotine stains off.
The house was built in 1957 - so we are assuming lead paint and yes I am taking it very seriously. We will be doing a lead paint test - it just hasn't happened yet.
I know for a fact that the current coat of paint was applied after 1990 when the previous owners of the house stripped all the wallpaper and repainted everything.
The walls and ceiling are drywall.
My goal is to paint the ceiling with a lower sheen paint, preferably flat. Is there a primer out there that will cover or might it be safe to scuff sand the latest layer?
So, any suggestions for dealing with my bi-sheen, bi-color ceiling? Thanks