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The cover stain is not too hard to apply, cleaning brushes is a pain though and it might curtain on walls. The 123 is a universal primer, I've never applied it on whole walls, the cost would be a factor. As I mentioned I had good results just washing the walls and one coat of emulsion recently.
Forgot to add that my concern would be coverage with one coat of anything, nicotine would stain through the gaps in a single coat of paint. That's why I would line it if it's really bad. It's not a pub ?
Had this problem on a job the other day. I used sugar soap and then it covered successfully with one coat of emulsion. It was only 1 years worth of smoking though. Zinsser Cover Stain is great and probably the best blocking primer but it's oil based and doesn't cover well so it would be a pain to cover entire walls. I use it to block water stains on ceilings which it does very well.
I would try sugar soap and a high solids emulsion Dulux Trade Super Matt or similar although nicotine can stain through any emulsion over time. I would probably line with a thin lining paper rather than paint the whole room with cover stain. You need to wash it with sugar soap whatever, you could wash it then sand it and wash it again.
Bullseye 123 would be the easiest to paint on, I usually use that as a primer on existing paint, it would probably block nicotine better than emulsion. You need to stir the pot the whole time as the solids fall to the bottom of the tin while you are painting.