Undercoat Plasters
Monday, August 6, 2007 at 10:44PM If only everything in life was as straightforward as a skimcoat of multi finish! In anything other than a refurbishment or new build there is a strong possibility that the background will not be characterised by the perfect flatness and predictable suction of fresh plasterboard. This page will explore the options for surface preparation prior to the skimcoat. It may not have the glamour of the topcoat, but the undercoat is equally important.
Bonding
Sold in 25kg purple labelled bags, British Gypsum Bonding Coat is a very useful product. It has large particles of vermiculite in the mix that mean it can be applied in thick layers without slumping. Maximum thickness is 25mm, applied in two or three consecutive coats. Common uses are as follows:
- Forming shapes, for example when using arch forming kits.
- Replacing small areas of render prior to a top coat
- An undercoat on low suction backgrounds such as tiles or concrete (used in conjunction with a bonding agent such as PVA)
- A first coat on Artex ceilings to cover the patterns
- An undercoat on an unsound wall. As the bonding dries it stiffens the wall and gives a more solid base for the topcoat. Particularly suitable for patching lath and plaster walls when lime putty is not available.
Hardwall
Supplied in 25kg red labelled bags, Hardwall is less versatile than Bonding. It is for use as an undercoat on masonry. It has a controlled set, unaffected by weather but it is designed to dry relatively quickly. There is not much time to tinker with the surface so application needs to be prompt and succinct. As the name suggests it gives a hardwearing surface, perfect for high impact areas and ready for decoration once skimmed with Thistle multi finish. It is a direct alternative to sand and cement render Once dry it has only half the weight of render, and only needs mixing with water to be ready for application.
Applying undercoat plasters
Top coat plaster is only applied to a 2-3mm thickness. It cannot disguise major deviations in the underlying surface. Care and time need to be taken with application. Plastering is made so much easier when the preparation is sound. A good undercoat makes a world of difference to the prospects of a decent topcoat.
If putting Bonding or Hardwall onto a wall across a large area there are three stages to the process.
- Apply. Undercoats work best if stiffer than finish plaster because they go on at a greater thickness. Using a plastering trowel and a float, the plaster should be spread onto the wall at an ideal depth of 13mm.
- Once the wall is covered it needs to be ruled with an aluminium feather edge or derby. This is a piece of metal about six feet long and is a good way for a novice to achieve a perfect finish. Always start at the bottom and move the derby up the wall. This will drag the plaster away from the high spots and into the hollows.
- Once ruled off, return with the trowel for further smoothing and finishing. As the plaster begins to dry, lightly scratch the surface to provide a key for the topcoat.
Bonding can be used in many more ways, for example when forming arches or curves. It is relatively easy to use after some practice, but there are too good tips.
- Be patient and apply in layers when a thick coat is needed. It sets in a couple of hours so any job should be possible in a day.
- For curves and rolls consider a small trowel such as a gauging trowel. Narrower, and tapered, they are much easier to use in tight spaces than a full sized plastering trowel.
Undercoat plasters are easier to use than a topcoat because the finish is not so important. Many people are deterred from plastering because they feel awkward on the tools and get disheartened when they cover the floor instead of the wall. The stickiness and stiffer mix of undercoats make them a useful starting point for getting used to using a hawk and trowel. Don’t be afraid to give it a go, and remember you can always get a pro in to do the finishing if it goes wrong!
Reader Comments (1)
Thanks for the inspiration this morning my friend. I coated two rooms (old plaster in ok shape) with thistle joint cement, i'm really happy with the result, but could've done without the dust.I tried one wall of another room with thistle one coat, but my plastering chops weren't up to it, and needless to say i was getting a bit pissed off with the whole thing. i bought some bonding,(used for patching aformentioned rooms), and thanks to you am inspired enough to go and get it on. all the best.