Plastering Techniques

Filed Under: Do it yourself, Home repair, Remodeling    by: ITC

Plaster is an excellent and inexpensive material for giving a smooth, hard surface to an internal wall so that it is ready for painting or wallpapering.

There are many types of plaster, but they can be divided into two basic types: gypsum-based and cement-based. The former are used solely for indoor work, whereas the latter are mainly used outdoors for rendering walls. Cement- based plasters do have a use indoors. however, and that is to finish external walls that might be subject to damp penetration; damp will attack a gypsum plaster and cause it to crumble.

Modern plasters come premixed with lightweight fillers such as perlite or vermiculite, which give a higher degree of thermal insulation and fire resistance and should be mixed with clean water.

Plaster is normally applied to the wall in two layers. The first, called a “floating” coat, is intended to even out the irregularities in the wall surface, so it is kept fairly thick — about 3/sin being usual. The second, finishing coat is spread much thinner Ysin or so — and carefully toweled off to a smooth finish.

Different types of building materials absorb water at different rates and if too much water is absorbed from the fresh plaster, it will dry too quickly and crack.

For example, bricks and lightweight building blocks absorb water quickly and are termed high suction surfaces. On the other hand, materials such as concrete and gypsum board do not absorb water that quickly and are termed low suction. You must choose a plaster to match the surface; but if in doubt, the best thing to do is coat the entire wall with a bonding agent which will make a low suction surface.

Browning plaster should be used for the floating coat on high suction surfaces and Bonding plaster on low suction surfaces. Finish plaster can be used for the finishing coat in both cases.

Only buy plaster as you need it since it has a limited shelf life. A 22lb bag of Browning or Bonding plaster should cover an area of about 1.8yd2 at a depth of 3/sin. The same quantity of Finish plaster, spread thinly, should cover an area about 6yd2.

In addition to a couple of clean buckets and a long level, you will need some special plastering tools: a spot board about 3ft square and supported on trestles or an old table to hold the mixed plaster while you work; a hawk for carrying small quantities of plaster to the wall; a rectangular metal plasterer’s trowel: a wooden float for producing flat surfaces (with a few nails knocked into the end it can double as a “scratcher” for scoring the floating coat before applying the finishing coat): and a 5ft length of 1 x 3in planed wood for leveling the plaster surface.

Cleanliness is essential when mixing plaster. since any dirt present in the mix will affect the drying time. Always use clean tap water for mixing and have a separate bucket of water for cleaning the tools as you work.

Mix the plaster and water in equal volumes in a clean bucket, adding the plaster to the water by sprinkling it on top and breaking up any lumps between your fingers. When the water has soaked into all the plaster, use a thick piece of wood to stir the plaster into a smooth consistency, (Finish plaster should resemble runny ice-cream). and make sure there are no lumps.

Wet the spot board and turn out the plaster on to it, kneading it with the trowel. If the mix appears too wet, sprinkle on a little more plaster and mix in with the trowel.

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