Hello Im sorry it has taken so long for someone to get to you.
I hope that I can give you some help.
Before you go to the expense of getting it boarded and plastered. Perhaps you could try this. Get some PVA mix it into a faily stiff solution and give two coats to seal the ceiling.
When this is dry after about 8 hrs. Then get some oil based base paint. put a couple of coats of that on. If this seals the smell you can coat it with a finish coat.
I hope that will be successful for you.
Hi Sorry to hear about this issue.
The best thing to use is a product called KILZ.
You can get that in an oil base but that is not necessary as it is also available in a water base and it works excellent for mold, mildew and smell coverage from other sources.
I would however go back to the place of purchase for the paint you used originally to ask them what could be wrong with that paint.
They will likely(after hearing your dilemma) supply you the KILZ or similar product free of charge. Tell them you want your money back as well. I have been an expert in issues like this for over 30 years and I will stick with you on this but since this question may continue for some time, if you will rate my service as good or betterbefore you close this window, the system here will allow me to schedule free follow ups with youso your question wont close or time out and you can contact me anytime at no additional cost.Thank youTech Michael