Internal Plastering Plastering helps provide better sound and thermal insulation and fire resistance. A good plasterer should have a sound knowledge of materials to use and their characteristics. A plasterer should identify the correct materials and their suitability for any given situation. A good plasterer should be able to put right any problems with their work and that of others.
BACKGROUNDS The background is the surface that your first coat of plaster is applied to. The background needs to provide a good bond for your plaster. Preparation of backgrounds is key to a good standard of finished work. If a background is not suitable or prepared well, your plaster will not stick to it properly.
SUCTION A plasterer needs to check the rate of suction of the background before applying plaster. Too much suction can cause the material to shrink or crack and loose adhesion. It can be difficult to rule in or get flat because of the loss of water in the material. The plaster will not reach its full strength and could fall off. Too little suction can cause the material to slip down the wall making it hard to rule in.
High Suction Background
Can include Aerated Block, old brickwork and softer types of brick. Can be wetted before plastering to control suction. Can be controlled with two coats of PVA , one very wet to seal and a 50 / 50 mix for adhesion. Use direct bond, hardwall, render.
Medium Suction Background
Can include lightweight aggregate concrete blocks and clay brickwork. Generally has good suction. Make sure is dust and grease free and clear of mortar snots. Can use all plaster products.
LOW SUCTION Includes in situ concrete, plasterboard, painted surfaces.
Never apply render until a good key has been formed or it will fall off. Use spatterdash, gyp bond for key. Use Carlite bonding, direct bond.
EXPANDED METAL LATHING
Can be used inside and outside. Flexible so can be curved. Fixed to wall plates to provide a key and resist movement. If fixing rib lath the ribs should be in contact with and at right angles to the support. Needs pricking up coat first. Use Carlite Bonding.
A background with a range of materials. This can have different rates of suction. Use different methods to create a key eg pva, gyp bond, spatterdash.
PREPARATION 1. Brush down the background to remove all loose materials and dust etc. to give a good sound background. 2. Remove all mortar snots from joints, internal and external angles. 3. Make a key, if needed to the background. 4. Test the background for suction ( splash some water on the surface and see how quickly it gets sucked into the background). 5. Treat if necessary.
PREPARATION 6. Place drop boards at bottom of wall to catch any material that is dropped. This can then be re used. 7. Fix EML to any timber wall and sole plates as necessary. This stops the plaster cracking and falling off the wood. 8. Fix any angle beads that are required. 9.Set up the spot board and stand.
METHODS OF WORK BROAD SCREEDS. Use on small areas.
Does not give the highest standard of work but better than freehand. Apply screed to corner of wall and rule off. Keep depth of material to no more than 11mm. Place another vertical screed in other corner and rule off.
BROAD SCREEDS Don’t place any screed in between the two end ones.
Form a horizontal screed top and bottom and rule off using the vertical screeds. Using the screeds to rule off, fill in between the screeds.
DOT AND SCREED Can be used on long or high walls.
Use if the finished work has to be absolutely plumb. Gives the highest standard for tolerance purposes.
DOT AND SCREED 1. Place a dot of material near an internal angle, place a lath in the dot. Dot should be about 300mm away from adjoining wall. 2. Place a second dot at the bottom of the wall, place a lath into the dot. Dots should be about 2.5m apart. 3. Plumb in the two dots using a floating rule. 4. Either tap in or bring the wood out until the two dots are in line.
DOT AND SCREED 5. Place another two dots at the other end of the wall.
6. Place intermediate dots in the middle of the wall. 7. Line the intermediate dots through horizontally off the two perimeter dots. 8. Join the dots together as the Broad screed system. 9. Fill in between the screeds and rule off.
APPLYING A KEY Each progressive coat of plaster needs to be keyed.
A key provides an Inter Surface bond. The depth of key used depends on the weight of the next coat of plaster to be used.
After the wall has been keyed before skimming run the trowel over the wall to remove any pick ups etc. All internal angles need to be ‘cut in’ using the trowel. Any plaster along external angle bead needs to be cut back to allow room for the skim coat.
FINISHING The finish coat provides the final, smooth coat for decoration. Generally not more than 3mm thick but can be up to 5mm. Always applied in two coats for the best finish result. If matching to a lime plastered surface the finish used will be lime putty / fine sand mix. Finish coats should not be over trowelled as this can prevent the paint sticking to it.
METHODS OF WORK TROWEL,TROWEL
This is the most commonly used method of work for skimming. 1. first coat is the scratch coat. 2. each stroke should overlap the previous one. 3. fill any hollows, joints etc. with the scratch coat.
TROWEL,TROWEL 4. Once scratch coat has picked up – apply laying down coat. 5. this should be slightly tighter than scratch coat. 6. When laying down coat has picked up enough, apply first dry trowel. 7. Trowel twice more with just enough water to lubricate the trowel and to work the plaster.
TROWEL,TROWEL Any hollows, marks etc. should be filled with the first trowel (fat). Any later than this the fat will be too watery and has no strength. 8. Clean angles out at final trowel and use brush to clean angles and beads. 9. Cross trowel at right angles to trowelling strokes to remove any water or fat marks. 10. Clean skirting line of excess plaster material (lips)
TROWEL,FLOAT,TROWEL This method is useful on uneven backgrounds.
1. Apply scratch coat with trowel. 2. Apply second coat using a float. Because the float is more rigid than a trowel, it will straighten out any irregularities in the floating coat. 3. When the second coat has picked up, apply thin laying down coat with the trowel. 4. Trowel the work as before.
BEADS Float ( angle ) bead. Used to form external angles.
Galvanised or stainless steel. Fix using dabs of material. Need to be plumb, level and straight along their length. Make sure wings are behind finished edge.
STOP BEAD Reinforces and finishes edge of plaster where no other material meets it. Can be used to finish plaster if butting up to another material if there is no cover being used eg. No arcitrave.
FEATURE BEADS Used to form shadow gaps along skirting board.
MOVEMENT BEADS Fixed over any movement joints in the background to prevent cracking of the plaster. Fix with dabs. Can be formed with two back to back stop beads and mastic filler.
MATERIALS Carlite Bonding
Undercoat plaster for low suction backgrounds eg. Concrete and plasterboard or surfaces treated with a bonding agent. Purple bag.
THISTLE HARDWALL Undercoat plaster with high impact resistance.
Quicker drying surface. Use on medium to high suction backgrounds. Blocks, bricks,thermalite.
THISTLE TOUGHCAOT Undercoat with good impact resistance.
Use on medium to high suction backgrounds.
CARLITE BROWNING Undercoat for backgrounds of moderate suction with a good key. Make sure suction is controlled when applying.
DRI COAT A cement based undercoat for re -plastering after installation of damp proof course.
LIMELITE RENOVATING Lightweight cement based backing plaster.
Contains waterproofing and salt inhibiting additives. Used in re- plastering after installation of damp proof course or in the renovation of old solid wall properties.
BOARD FINISH Finish for low to medium suction backgrounds.
Plasterboards, Dri Coat. Class B hemi Hydrate.
MULTI FINISH Can be used on all suction backgrounds.
Good choice where both solid and plasterboard backgrounds are involved.
DURA GRADE Specially formulated to produce a hard finish.
Used in high traffic areas. Can be used on plasterboard, Dri Coat, Gypsum backgrounds and cement based backgrounds.
HIGH IMPACT FINISH High impact finish is used to finish Limelite products. This is a Class D anhydrite plaster. Gives a very hard and high quality of finish. All of the water is driven off when the powder is produced. The plaster crystals take longer to form when the water is added when mixing. An accelerator is added to kick start the reaction. The slower forming crystals give a harder more durable finish coat.
RE CAP Aerated blocks and old bricks are ………… suction backgrounds.
Concrete and plasterboard are ……….. suction backgrounds. If plastering to a low suction background Carlite …………… should be used. If re plastering after damp proof work the backing coat should be ……………….
RE CAP After forming the perimeter dots on a wall the intermediate dots should be …………… in. Internal angles should be ………. in after floating. Different types of key are used dependant on the ………….. of the subsequent coat. Angle beads should be fixed to concrete with dabs of Carlite …………………..
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