Plaster blows because it either has no, or has lost its, adhesion with the surface it has been applied to. This can be for a variety of reasons the most common of which are 1. The surface its being applied to is very porous. The water is sucked out of the new plastyer before any chemical reaction has had a chance to take place. The plaster still goes hard, but does not "stick" to the surface. THis can also happen with paint if its applied to new plaster for example, this is why the wall is sealed with a diluted coat of paint and a similar sealing process should be done before platering on a porous surface. Thye use of PVA is not uncommon but if the surface is not cleaned first and has elements of grease or dust on it, the adhesion of the sealer will be poor which makes any adhesion of the plaster to the surface impossible. In 30 years we have only ever had a case of "bad" plaster once and that was only because it had been stored in a damp place which we were unaware of. This is not to say bad plaster does not happen but it generally becomes apparent while the plaster is still in the bucket.
Last edited by thedoctor on Mon Apr 30, 2007 5:01 pm, edited 1 time in total.