OK two options.
1 Paint two coats of Classidur no cleaning or washing just scrape or dust off any loose stuff and follow the instructions on the tin about re-coat times. Job done, only snag is the price of the paint will make your eyes water! The good point is that is your finish coat and it is easy to do. http://www.decoratingdirect.co.uk/viewprod/c/CLASSIDUR_SUPERCLASSIC/
2 Traditional way, wash with sugar soap (wear eye protection to do a ceiling) rinse, then two coats of stain block (I swear by zinsser ) http://www.decoratingdirect.co.uk/viewprod/z/ZINCS/ Then apply two coats of a quality emulsion (Dulux Trade) stand back and admire!
Wow that Classidur stuff sure is pricey. If it works as well as footprint says though, it'd be money well spent. I was interested to read about it.
I've used Zinsser BIN bulls eye 123. It's water based and although it has looked like it hasn't worked very well, when you emulsion on top, you see it does actually work. It's half the price of the Classidur so still not cheap, but cheaper..
I've used oil undercoat and it will work but for me personally it's stinky, not all that cheap, you'll do in a roller putting it on and you might need two coats of it and still two coats of emulsion...
I'd say it all depends on the amount you want to spend, the area needing done and the time and effort you want to put in.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local