Sorry but with over 20 years experience in fire cleaning including reporting on ballsed up jobs, sealing alone is just not adequate.
There are 2 types of smoke residue, wet and dry. Dry residues are the result of cellulosic materials burning such as wood or paper. This usually occurs when say a waste bin catches fire. In most domestic fires there is usually a mixture of substances which have burnt. This includes plastics which give off Polyvinylchloride when burnt. When mixed with the moisture in the air this creates Hydrochloric Acid. This Acid is what causes most of the damage as it discolours plastics and decays metals. This needs to be neutralised using a high alkaline detergent.
The excess soot needs to be removed before any sealing takes place. I have been to many failed jobs where builders have just sealed the soot, even with Zinsser, and the odour has returned.
The easiest way to remove the excess soot is to put a high alkaline detergent in a spray bottle, spray the wall and remove with a terry towel cloth. Use a clean cloth as soon as they become black. Do not try and clean using buckets of water because as soon as you wring the cloth out you have a bucket of oil and you just re-contaminate the wall.
We have all the products available and we also have a fire procedures sheet we can email.
http://www.restormate.co.uk/epages/15094.sf/en_GB/?ObjectPath=/Shops/15094/Categories/Fire__Flood_Restoration/Fire_Restoration_Products