Best way to paint over yellow stained ceiling

[Sprocker]

Hi
My house suffered a roof leak several years ago when the previous owner was still living here and the ceiling now looks very marked and yellow
I've just painted the ceiling with emulsion and it basically looks the same
I painted the other bedroom ceiling last year and it took several coats to start looking more respectable obviously would like to avoid doing this again if possible
Does anyone have any ideas to seal the ceiling at all, I was going to use a damp sealer on the whole ceiling but unsure if this is the most economical way?
Thanks in advance

 

[KenGMac]

Sprocker, good evening.

Suggest you look @ paints marketed as "Stain Block"

Or?

Another way would be to overcoat the entire ceiling with a "Flat Oil Based Paint"

If the ceiling is dry then I would not bother with damp sealers.

Ken