I would clean the spot using TSP (trisodium phosphate is a
good degreaser cleaner). Then use shellac to seal the spot if there is
still any discoloration.
It may be a good idea to prep the kitchen ceiling with a TSP
washing to get rid of any cooking vapours that may have deposited a
film of oil and dust. Paint won't stick to dirt or oil.
P.S. I used TSP to remove the grey woodstove smoke buildup from a
ceiling when all other cleaners failed. TSP is excellent, just don't
forget to rinse and used rubber/vinyl gloves.
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