PREPARE BACKGROUND SURFACES AND PLASTER MATERIALS DEVELOP AN

PREPARE BACKGROUND SURFACES AND PLASTER MATERIALS

DEVELOP AN UNDERSTANDING OF DIFFERENT TYPES OF SUCTION ON BACKGROUNDS, AS WELL AS THE MECHANICAL KEY THEY WILL HAVE

PREPARE BACKGROUNDS Introduction Aims: To understand the different backgrounds to which plaster is applied Objectives: To know and recognise different backgrounds To know how to prepare different backgrounds 3/

PREPARE BACKGROUNDS What is a background? A background is any surface which plaster / render can be applied to Examples: n Brickwork n Blockwork n Concrete No background, no plastering 4/

PREPARE BACKGROUNDS Introduction Can you answer the following? n What is suction? n How can suction be controlled? n How can suction be created? n What is a mechanical key? n What is a bonding agent? 5/

PREPARE BACKGROUNDS Backgrounds If a background is porous, it will absorb water like a sponge. If a background is dense, it will absorb little or no water This is called SUCTION The more water the background absorbs, the higher the suction rating. The less water a background absorbs, the lower the suction rating. Question: What affect will this have on the plaster applied? 6/

PREPARE BACKGROUNDS Backgrounds Gypsum plasters rely on retaining some of the water used to mix them until the setting process is complete, if too much of the water is absorbed by the background the plaster will shrink and crack and lose adhesion with the background causing bond failure. Suction needs to be controlled. How can you tell if a background is HIGH, MEDIUM or LOW SUCTION? 7/

PREPARE BACKGROUNDS Suction To establish how much suction a background has is quite simple. It’s as simple as splashing some water onto the background and watching how quickly the water is absorbed. The quicker the water is absorbed the higher the suction. Over time and with practice this will become quite a good indicator to you has to how much suction a background has. Try it! 8/

PREPARE BACKGROUNDS Background preparation High suction backgrounds Bonding agents are used to seal the background and control the suction to leave a surface suitable for plastering. Thermalite Blocks The most widely used are PVA, Thistle Gypprime and SBR. It is vital that the manufactures instructions are followed when using these products to avoid problems in the future. Easily applied using a roller. Porous Bricks 9/

PREPARE BACKGROUNDS Background preparation Low suction backgrounds Because plaster relies on some suction and in most cases some form of key this type of background needs the most treatment. Dependent on the type of background (tile, concrete) the easiest treatment is to coat the surface with a bonding agent such as Thistle Bond-It, which creates a key and creates an even suction throughout the background. 10 /

PREPARE BACKGROUNDS Background preparation Different backgrounds require different treatments before we can apply any plaster. Treatments vary from: n Damping down n Bonding agents n Scratch coats n Hacking n Raking out of joints 11 /

PREPARE BACKGROUNDS Background preparation Creating key Mechanical key Hacking This can be done using mechanical or manual means and is done to provide a key for the plaster to bond to low suction background Racking out joints Again this can be done both mechanically or manually 12 /

PREPARE BACKGROUNDS Background preparation Creating key Mechanical key Scratch coats Applied using various materials depending on circumstances, can be used to create key, suction or to even out suction especially on composite backgrounds. Bonding agents are easily applied using a roller, and are used to seal the surface and with some bonding agents to create key as well 13 /

PREPARE BACKGROUNDS Background preparation Creating key Mechanical key Expanded metal lath Can be used to create key on timber lintels or timber wall plates, and is very useful to overcome situations involving composite backgrounds. 14 /

PREPARE BACKGROUNDS Background preparation Creating key Natural key Is the open texture found on some backgrounds such as breeze block and some brickwork. 15 /

PREPARE BACKGROUNDS Conclusion You should now be able to answer the following: n What is suction? n How can suction be controlled? n How can suction be created? n What is a mechanical key? n What is a bonding agent? 16 /

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