Whilst the full scope of plastering is a set of skills learned and practiced over an apprenticeship of many years, there are some useful aspects of plastering that can be learned quickly. With on going practice you will be amazed at how quickly you are able to produce results that you would be happy to pay for or maybe even be paid for!
In this DIY guide we take a look at how to prepare for skim coat plastering which includes how to fix plasterboard to a stud wall or dot and dab over an existing surface, repair areas of plaster, lay on undercoat plaster and then apply undercoat plaster.
What Jobs Can a Novice Plasterer Tackle Fairly Easily?
The types of jobs that one can tackle confidently with a small amount of training include the following:
- Applying Plasterboard to a wood frame, stud wall or ceiling with nails or screws. More can be found on this in our plasterboard project here
- Applying plasterboard to solid walls using plasterboard adhesive known as “dry lining” or “dot and dab”. More can be found about this in our dot and dab project here
- Skimming a plasterboard surface and re skimming an entire room including a ceiling ready for redecorating
- How to repair areas of plaster from bare brick with a patched skim finish
However, one thing that you will definitely need to appreciate is what plastering “feels” like plus how to read and maintain control of the surface that you are plastering. Without expert tuition the probability is very high that you will produce a finish that needs to be completely removed or covered over by a paid expert. It is very likely that the cost to repair in terms of money and or time will far out way the cost or attending a short plastering course.
A good quality finish is the result of managing the tools and materials in response to the changing conditions of the plaster that you are applying and as you can imagine, much of this is down to experience gained through working in this area for many years.
What is Plasterboard?
Plasterboard is a material that enables solid and stud walls to be prepared for skimming in a fraction of the time and cost that it would take to float a wall using the traditional wet wall plastering methods. It also enables novice or DIY plasterers to plaster newly built walls or to make walls square and plumb for re-plastering or tiling.
Plasterboard comes in a range of sizes and thicknesses to suit the scale of the task being tackled. Plasterboard comes with a range of characteristics that relate to the nature of the application, these include:
- Standard plasterboard usually 9.5mm or 12.5mm in depth and in a range of sizes
- Fire lining plasterboard used to increase the fire resistance of a wall
- Moisture barrier board for areas such as bay windows were warm moist air meets cold air causing condensation
- "Aqua" board used in wet areas such as tile backs for showers and bathrooms
- Sound and thermal insulation boards helping to meet the specifications of local building regulations
Always discuss the application with your builders’ merchant to ensure that you are purchasing the correct plasterboard type for the job
Plasterboard is typically cut to size prior to fixing. If applying plasterboard to a stud wall you will normally cut to the centre of the joist or noggin. If applying plasterboard to a solid wall you will usually cut the plasterboard to the appropriate size to maximize the efficient use of the board that you have purchased.
Cutting Plasterboard to Size
To cut plasterboard you will require the following tools: Retractable Knife, Straight Edge, Tape Measure, Pad Saw, Rasp and a Pencil.
Measure the dimensions that are required and transfer those to the plasterboard. If new to this, mark the line with a pencil and always “measure twice & cut once”. If you are able to make a cut across the complete plasterboard width or length then run the knife along the marked line using a straight edge as a guide ensuring that you have cut one side of the paper. Apply pressure to the reverse of the line that you have scored and you will find that the board breaks cleanly along that line. Now run the knife along the unbroken sheet of paper and you will have completed your first cut.
The rasp can be used to tidy up the cut or shape the cut to fit non straight edges.
To cut a recess, around a window corner for example, into a piece of plasterboard you can use a combination of the pad saw and the knife. Make the initial short cut into the board with the pad saw and then repeat the cutting process from the edge of the board to the end of the pad saw cut using the straight edge and the retractable knife.
To fix plasterboard to a stud wall use galvanized clout nails or specific drywall screws. To apply plasterboard to a solid wall use the plaster board adhesive and the dot and dab method mentioned above. More on fixing plasterboard can be seen in our project here.
When applying board to a ceiling make use of a “third arm” or “dead man” which will hold your plasterboard in place whilst you fix it to the ceiling joists.
Dry Lining or Dot and Dab
This is the process of applying plasterboard to a solid wall. Having cut your board to size, apply dots of plasterboard adhesive to the wall. Ensure that the edges of the board are well supported with adhesive especially where the skirting board will be fixed.
Using a straight edge and a level make sure that the plasterboard is vertical and straight by gently tapping the appropriate area of the board with the straight edge. Also make sure that the board is not diving into or out from the wall on the horizontal.
Use the same method to apply the subsequent pieces of board but run the straight edge across multiple boards to ensure that they are “true” and “plumb” in relationship to each other. Failing to do this will result in a wall that undulates along its length and has steps between each board resulting in a surface that is difficult to skim.
Skimming and Over Skimming
Skimming is the process of applying a finishing plaster skim coat over undercoat plaster that has already been applied to the wall. The skim coat is appied to teh undercoat and fully smoothed to create a flat and level finished surface.
Over skimming is the process of applying a final skim coat over an existing finished skim coat. When an existing plastered wall becomes damaged and chipped or is horizontally or vertically un-level it is often much easier to put a finished skim coat over the entire surface to sort it out.
We recommend that Multi Finish is used by the novice plasterer. Board Finish is usually used when skimming on to a purely plasterboard surface but Board Finish goes off too quickly for the novice.
Tools Required For Plastering
Here follows a list of the common tools and materials that are used when plastering:
- Flexible bucket
- Bucket trowel
- Spot board
- Hand board
- Finishing trowel
- Clean water
- 1” and 4” clean paint brush
- The correct type of plaster for the job you are doing
The Plastering Process
Having applied scrim tape to the joints and nail heads of the wall that you are about to skim, mix the correct amount of plaster to a smooth creamy consistency making sure that ALL lumps are out and dry patches around the mixing bucket are mixed in. Having pre wet the spot board pour the plaster on to the spot board.
Having also wet your hand board and trowel, prime the hand board with a small amount of plaster and then load the plaster on to the hand board. To transfer the amount of plaster from the hand board to the trowel pass the trowel over the plaster on the hand board and “squeeze” the plaster onto the trowel creating a “sharp” wedge of plaster on the edge of the trowel.
Dividing your wall or ceiling into three notional horizontal sections, start plastering at the top left of the top 1/3 of the wall moving across to the bottom right of the of the top 1/3 of the wall.
Apply an even coat of plaster across the area of about 3mm in depth.
Once you have covered this top 1/3 move down and repeat the process on the middle third. Once that is similarly covered repeat on the bottom 1/3.
The “pattern of three” process described above is to enable novice and experienced plasterers alike to maintain control of their wall and predict the way in which the plaster will go off.
It also helps you to know where you started using a new mix if you run out of plaster half way through a wall.
Now smooth this first coat so that edges and corners are “sharp” using the wet small brush or edge of the trowel.
Also, now smooth the wall following the “pattern of three” to a relatively smooth finish. Basically you are removing raised aspects that are more than 2 mm high and filling any holes in. Typically you will do this by moving the plaster around that is on the wall, essentially moving the high spots into the low spots.
Now make a second mix of Multi Finish and apply this plaster coat to the wall. This is the laying down coat. The first coat of plaster was called the roughing on coat. Follow the exact same “pattern of three” and this time aim for a smoother finish but without delaying the process of getting the entire wall plastered. Remember that plaster will only remain workable for between 10 and 20 minutes prior to putting it on the wall.
Once you have covered the wall you can now do a first trowel on the wall to get it smooth. Again follow the “pattern of three” and avoid concentrating on a single spot. Make sure holes are filled and lines flattened moving towards a finish similar to that which you want to finally achieve. This is really your last chance to get the smooth finish that you require removing large lines and holes!
You are now going to trowel the wall in three sets of three trowels. The first set of three trowels is when the wall is becoming firm and you should trowel the wall with single strokes of the trowel three times.
Up to 15 minutes later and using a moist 4” paint brush, brush the wall ahead of the trowel during the 2nd set of three trowels. This will help to lubricate the passage of the trowel over the drying plaster and prevent dragging.
Now up to 40 minutes later when the wall feels as hard as it does when set you can use the same technique as in the 2nd trowel above to give a final finish to the plaster.
This set of three trowels is done in the “pattern of three” and enables you to maintain a consistent finish across the wall, avoid concentrating on a single area to the determent of the rest of the wall and stops the wall becoming over troweled. The final finish should be dull in appearance yet smooth to the touch.
Over Skimming a Wall
The process of over skimming a wall is identical to that of a plasterboard wall except that the wall needs to be sealed by the use of diluted PVA glue to prevent it sucking all the moisture from yor new top coat beofre it has dried prior to the application of the plaster and any loose plaster should be removed and repaired as in the process below.
Dilute PVA glue 5 parts water to 1 part glue. Again, using the “pattern of three” brush up to two coats of PVA on the wall. Once dry this will inhibit the rate at which the old plaster draws moisture from the new plaster and slows the rate at which it goes off thus extending the time that you have to work with the plaster. N.B. There is no known additive that inhibits the rate at which plaster goes off so don’t try and add PVA to the plaster and expect predictable results.
You may want to “feather in” the plaster upto window beads unless you are prepared to apply new beads which will need to be done with plasterboard adhesive or bonding.
Patch Repairing Plaster
When a patch of plaster has come off part of a wall it is possible to first of all repair the patch prior to skimming with Bonding Compound. Apart from the mix typically being firmer that that of Multi Finish the process of mixing to getting it on the trowel is identical.
Divide the repair area into two horizontal parts. Starting at the top right, plaster across the top half. Where necessary build up the bonding in coats of no more that 15mm to avoid slumping or cracking. Now do the same on the bottom half. Build the coats up until the top half is just proud of the repair area.
Now, glide a plasterers darby across the patch bridging the darby across the old plaster to give a level. Use the darby to spread and grade the plaster. Now complete the final coat of the bottom half of the repair and repeat the process with the darby. You should now have an area of bonding compound flush with the old plaster.
Give the bonding one trowel to smooth. Any more will make the plaster too smooth and your skim coat will blister. Once dry you can re skim the wall as above. If you are going to feather in a patch repair leave the bonding just a couple of mm shallow of the surrounding old plaster and fill the rest with skim.
Plastering is a very difficult process and plasterers train for a long while to get a good, smooth flat surface. No matter how many projects you find or how many books you read you will not be able to just walk up to a wall or ceiling and get a good job. Practice is the only way.
Books are a great way to learn "plastering rules" but there is no better way, in fact there is no other way, to learn to plaster properly than by actually doing it.
Plastering is about timing and "feel". A wall can look flat, but a plasterer can run a trowel over it and know exactly where the imperfections are, and, more to the point, put them right.
Make no mistake, plastering is a skill equal to an electricians or a plumbers and if it is not done correctly the first time, can cost a fortune to put right.
For this reason alone we recommend enrolling onto a reputable plastering course and at least mastering the basics to give yourself a head start.
For those who want a go themselves the basic rules are:
The first thing we recommend strongly, is to have a practice before you start spreading plaster all over the place. Plaster, in scratch coat and top coat form, goes off very quickly. You do not get a lot of time to work with it and get it smooth.
If you do not have the back of an old shed wall to practice on, buy a sheet of hardboard from your local builders merchants. Paint this with an undiluted coat of PVA adhesive (see our project on PVA) Let this dry for 10 minutes and stand the board up against a wall, tacking a couple of nails in to hold it upright. You can practice spreading on this surface and getting the hang of transferring plaster from the hawk to the board.
Laying on Undercoat Plaster
The application of undercoat plaster to the wall is called ‘laying on’. Use angle beads where required and see our project on base coat plastering.
You will need:
- a bucket of water
- a large emulsion brush
- undercoat plaster mix (see Chapter 9 for details on mixing the plaster)
- a hawk
- a bucket trowel
- a steel trowel/float
- a straight-edged length of metal 1.5m long
- a plastic float
- a small tool for filling narrow gaps (see figure 9.1)
- a nail or a small screwdriver to ‘scratch’ the plaster.
Before you start to apply plaster, damp the wall down with a large emulsion brush and water. Don’t let the water run down the wall – use just enough to make it damp.
How to Apply the Undercoat Plaster
The mixed plaster should be placed on your hawk about two trowels full at a time.
It is then scooped from the hawk to the steel trowel (sometimes called a steel float) on to the wall. Spread it about so it is slightly proud of the beads on the wall.
Then take a straight edge (metal is better than timber) and lay it over the two beads at the bottom of the section you have filled. Push the straight edge against the beads and pull upwards, sliding side-to-side as you go. Do not worry if some chunks come out as you drag upwards.
Scrape the plaster off the straight edge back into the bucket and go back to your hawk and trowel. Fill the holes and any areas that may be a little low. There should be some surface fissures, but not too many and none too deep.
- Repeat the process a couple of times until the section is full, flat and relatively smooth, then move to the next section.
- The metal beads are galvanised (so they don’t rust) and will stay in the wall. There is no need to remove them. When you have done two or three sections, the plaster should be beginning to go hard
- When you can put your finger lightly on the surface without leaving a mark, it is time to ‘rub in’. This means getting the plastic float and rubbing in a circular motion over the wall to close any fissures still remaining
- You will feel the surface go smooth under your touch and after a while you should be able to sense any depressions or high spots in the wall. You can add, or remove, a little plaster to correct these
- When you have finished you will have a smooth-looking wall. Any narrow areas such as the gap between a doorframe and an adjoining wall can be filled with a ‘small tool’
- If you have fixed angle beads to any external corners, the undercoat plaster should finish just below the tip of the angle. This will allow you to use the same bead to plaster your top coat to
- After a couple of hours, the undercoat plaster will be hard enough to scratch. Take a nail, or a small screwdriver, and drag it lightly, in a coil shape, over the surface of the plaster. The scratch should be no more than 1mm deep and all of the wall should be covered, with no more than 150mm between each of the scratches
There are special metal ‘combs’ for this, but these are not really necessary for the area that you are likely to be scratching – the novice should aim for a maximum of 4 m2 in one go.
Rub over the wall lightly once more with your plastic float, just to flatten out the burrs caused by the scratching.
Your wall is now ready to apply the top coat.
When working out how much plaster to buy, you need to know that, at 12mm thick (approximately ½ an inch), 25kg of browning, bonding, or one coat plaster (scratch or base coat plasters) will cover just over 3 square meters. Top coat, or skim plaster should be applied at about 3mm
thick and you should get about 13 square meters from a 25kg bag. Do not buy any more than you need and do not store it for long before you use it. Plaster in all its forms, does not have a very long shelf life.