Building Guides Plastering

How to Apply Undercoat Plaster

apply undercoat plaster

When plastering walls, getting the undercoat plaster layer right is an important part of the process. This will be the base onto which you will apply the finishing plaster and needs to properly applied and sound or all the hard work you put into finishing the wall will be a waste of time. This guide will take you through the process of applying undercoat plaster on the average-sized domestic wall step-by-step. Take your time and work carefully, and even this often tricky task is perfectly achievable by the DIY-er.

Applying Undercoat Plaster

Applying undercoat plaster evenly over a large area can be difficult so it is usually a good idea to divide the wall up into sections using battens nailed to the block or brickwork. As long as the battens are all the same thickness, they can also be used as a guide for the plaster depth.

Preparing the Wall

Check the wall to make sure that there is no sign of damp or mold. If there is, this should be treated before you start to plaster. Using 10mm thick (30mm wide) battens cut to the full height of the wall,divide the wall up into 1000mm (1m) sections. Use masonry screws to fix the battens to the wall and a spirit level to ensure they are upright. Avoid using nails as the battens will need to be removed later.

Your wall should now be divided up into manageable sections that you can fill with plaster. Dry blocks or bricks will suck the water out of any plaster you apply to it and make the plaster dry too fast, so you now need to dampen the first section of wall you are going to be working on. You can use a wide paintbrush or a mister bottle to do this.

Mixing the Plaster

You will be mixing quite a lot of plaster, even for a fairly small wall, so it is worth investing in a stirrer attachment for your power drill. Follow the manufacturers instructions for mixing the plaster, being careful not to create a mix that is too loose. You will need a fairly stiff, even consistency if you don’t want the plaster to sag. Mix the plaster as you need it in a large plastic bucket. Don’t mix too much at once or it may start to go off before you can apply it to the wall.

Filling the First Section

With your plaster mixed and ready to apply, scoop a manageable amount onto a Plasterers Hawk. Using a clean plastering trowel, divide off some of the plaster on the hawk, tilt it and scoop the plaster up onto the face of the trowel. We are going to assume that you have at least some basic experience of plastering. If you have never plastered before, it is worth practising this movement a few times before you actually start.

Hold the trowel at an angle to the wall in the first section and carefully spread the plaster in a sweeping motion. You are aiming for an even distribution of plaster over the whole section, using the battens to guide you on the depth (10mm). Load more plaster onto the trowel using the same method (divide off some plaster, tilt the hawk and scoop upwards with the trowel to load it). Remember that you are aiming for a uniform depth of 10mm over the whole section. Undercoat plaster is quite forgiving, but you will still need to apply it to the wall with care.

Continue the process until the whole section is filled to a uniform depth. Using the battens as a guide, run a straight edge down the section to remove any excess plaster. If there are any areas that are too shallow, fill them up with more plaster now. Once this is done, repeat the pass with the straight edge.

Finishing Off The Undercoat

Repeat the process above for each of the sections on the wall, ensuring that each is filled level with the face of the battens. Unless you are plastering a very large wall, you should have plenty of time to fill all of your sections before the plaster goes off. When the plaster has started to harden, but still has some give in it when you press lightly on the surface, remove the battens that divide up the sections. Fill the gaps where they were fixed carefully, making sure that these plaster strips matches the depth of the surrounding plaster.

Before the undercoat plaster is fully dry, you need to score it to give the finishing plaster something to key onto. The easiest way to do this is to hammer several nails through a small piece of wood and then use this makeshift tool to score the surface. Drag the points of the nails lightly across the damp surface, being careful not to press the points too deeply into the surface. All you are doing is scratching the surface, not gouging deep tracks.

Make sure that you allow the correct amount of time for the undercoat layer to “go off” before you begin to work on the finishing plaster layer. Working on plaster that is too damp will result in problems with the finishing plaster later.

Applying undercoat plaster to the external corner of a wall (on a chimney breast for example) can be tricky. You will need to use Mesh Angle Bead to create a strong and straight corner. You can read our full guide to Plastering External Corners for more information.