One coat work is usually a finish coat. It is mostly skimming or setting coats. It is applied as a setting to cover materials like plasterboard and lath. One coat work is chosen for certain backgrounds because of the cost, weight and bonding risks.
You can make the cost less by using less material and finishing the work quickly. Bonding is less likely to fail with thin coats than thick coats such as two coat work and three coat work.
One coat hand applied plaster is often made of gypsum based plaster, perlite and other additive that improve setting times. This mix can be used on concrete, bricks, plasterboard and metal lath. If the background is high suction, a diluted solution must be used.
You will get the best results if the plaster is mixed in a large tub with water using a drill that has a whisk attachment. Always use the thickness that is recommended. On plasterboard it should be between 3 mm and 5 mm thick, concrete should be between 5 mm and 8 mm and bricks should be between 10 mm and 13 mm thick. Once the plaster has been applied to the background, it should be ruled off to make it even and then leave it to go hard.A trowel can then be used to make the surface flat. Once the plaster has been left even longer to set, a trowel should be used once more.
Two coat work can be used on all different backgrounds such as clay tile blocks, brickwork, concrete, plasterboard, masonry etc.
The results from using two-coat work will be good as long as the surface is quite even. If you are using two-coat work on bricks or stone and there are holes in them, the holes should be filled in first.
Two coat work is usually done by applying a floating coat, straightening and rubbing down and then then making a key on it. Once the floating coat has completely set, a thin finishing coat can be applied.
Three coat work is used on brickwork, stone etc. The rendering coat is applied to even out the suction for when the floating coat is applied. This can reduce or completely get rid of any differences in thickness and suction. Different materials in the background can be the cause for the suction difference.
Three-coat work can not only even out the suction, it can also improve the sound, thermal and fireproof properties in the structure.
When applying the rendering coat, it should be about 9 mm thick and scratched back with a wire scratcher to make a key. Leave the rendering coat for the amount of time recommended on the manufacturers instructions before you apply the floating coat.
The floating coat is usually straightened with a darby, then filled in using a trowel and rubbed down. When using the darby to straighten, it is better to make angles with a long rule first. The angles act as a guide for the darby to give a better end result.
Another way of doing this is to use floating screeds either vertically or horizontally but this takes longer. A strip of the mix used for the floating coat between 75 mm and 300 mm wide is applied along the width of the wall and is straightened using a floating rule. Once the screeds have hardened, the space in between the screeds is filled in stages about 600 mm wide. Once this has been applied the rule is used, making sure the edges are tight to the upper and lower screeds.
Another method is the plumb, dot and screed method. With this method, you will get a more accurate floated surface. You will need to take a small amount of the floating mix and apply it in the top right corner with a 150 mm gap between the ceiling and wall. A 75 mm lath should be put inside the floating mix that has been applied to the wall and the surface should be sticking out by about 12 mm. This should be done in all of the corners of the wall. The bottom laths should be directly in line with the top ones.
When both corner dots are perfectly in line, intermediate dots are put in. They should be roughly 2 metres apart. Screeds can then be made between the dots and can be used for ruling.
A good way of lining dots is to use three pieces of either 6 mm or 12 mm timber that are the same thickness, as spacers. A spacer should be put on each of the dots in the corners of the wall, and the line should held over these tightly. A dot is placed in to position where you need it between the corner dots. You will need to check that it is in a straight line by sliding the third spacer on top of the middle dot up to the line.