Alternatively, where cracks can be expected to form in the plaster over the junction of dissimilar materials, a straight knife cut through the plaster along the line of the junction will prevent the formation of unsightly irregular cracking.Where small widths of one material are involved, for example where a concrete column divides brick panels, the column width is best bridged completely by fixing expanded metal over building paper onto the abutting brickwork in order to isolate the plaster from differential movement.When plastering is to be continued across different backgrounds, expanded metal fixed across the junction will minimize cracking due to differential movements.It has many large voids and offers an effective key for plastering. A no-fines concrete is, on the other hand, an ideal material to plaster. Commence plastering once it is completely dry.It must be completely absorbed and that no shiny film remains on the surface. Apply Thistle GypPrime thinned with water depending on the absorbency of the background using a suitable roller, brush or spray.GypPrime prevents the rapid dehydration of undercoat plaster that can happen with high suction backgrounds.These conditions can present difficulties when plastering and bonding treatment such as a PVAC bonding agent or Thistle GypPrime may help. Fully compacted lightweight aggregate concrete has a different thermal movement value to plaster and, when hardened, can present a dusty surface with high suction.Plastering on Lightweight and No-Fines Concrete A disadvantage is that the method can be expensive. Probably the most risk-free option of plastering concrete is to apply the plaster to expanded metal lathing plugged and screwed to the concrete.On most new building sites, however, it is not practical to wait until the concrete has attained an air dry condition as this may take many months or even years in the case of very thick constructions.Plastering should certainly not commence if free water is present on the surface of the concrete. Concrete should not be plastered until it has had a reasonable time to dry out after the shuttering has been struck.
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