How to Mix Plaster, for a Perfect, Smooth Finish
Working with plaster can be a very rewarding and fun process, yes it’s very hard work but the sense of achievement I feel when I have covered a tatty-looking subbase with a smooth, creamy finish is immense.
But if you don’t prepare and get the materials right, plastering can be a nightmare, and after preparing the sub-base surface the next important step is mixing plaster correctly.
Mix plaster in a clean bucket with clean fresh water, fill the bucket with water first, then add in the plaster powder, use a hand drill and mixing paddle to blend to a smooth, creamy consistency, it should look like toothpaste or cake mix. Never add more water to plaster that has begun to set. Check the labels for mixing ratios.
Get this step right and the task of covering a wall is much easier and you won’t waste time re-working, sanding or even pulling the plaster off and starting again let’s look at the steps in more detail.
Tools and Materials
There are a few tools and things you will need for the job:
Preparing Your Work Area
Before you actually mix any plaster or ‘mud’, clear a space to work in, that won’t be in your way when you want to skim the surface that needs plastering.
Mixing plaster can be a messy job so project any surfaces with old sheets, newspaper or plastic dust sheets. Set a spot for you to clean or remove your work boots before you leave the area so as not to spread plaster across your home.
Keep a dustpan and brush near to hand and have a spare bucket of clean water nearby to wash your hands and clean your tools at the end of the job.
Make sure any old plaster is cleaned off your tools as contamination can cause fresh plaster to spoil.
It’s always better to clean your plastering tools straight after you have finished a job rather than before as it makes the process much easier if the plaster has not set hard.
Mixing the Plaster
When plaster is mixed to the correct texture and thickness, it is much easier to work with and apply, check the manufacturer’s instructions and read the labels on any bags,.
Always check the dates, as plaster will have a shelf life of around 4 months, that if exceeded, the mixture will not be strong enough to hold properly, and the working time can be compromised.
Plaster is usually sold in 50lb (25kg) bags, and quantities can be calculated against volume. As a general rule, gypsum finishing plaster should be mixed at a ratio of 2:1 ie 2 parts plaster to 1 part water.
So for a typical 50lb (25kg) bag, you will need approx 3 gallons US (11.5 litres). Use a clean bucket to measure the quantities and half bucket measures for smaller amounts.
Never mix up more plaster than you can comfortably work with, it’s better to have to mix up some more than have to throw a large amount away.
Always fill the mixing bucket with water first, then add the plaster, this helps keep dust levels down and means unmixed plaster won’t be stuck in the bottom corner of the bucket spoiling the mixture.
Add a small amount of dry plaster to the water and gently stir it in until it has fully dispersed then add some more,
You can mix plaster by hand but it’s a tiring job and can reduce the amount of time you have to work with the mud.
I prefer to use a mixing paddle or whisk attached to a small 18V power drill, my little Hitachi has loads of torque for this, and setting it on low speed is just right to get the mixture perfectly smooth.
A note of caution when mixing and using plaster never add more water to a mixed batch that is beginning to set, this will reduce the strength of the plaster and make it horrible to work with.
Only mix up enough plaster that you can apply in around 20 minutes or so, you will get better at judging this amount the more practice you have.
Mixing Undercoat Plaster
The best way to mix undercoat plasters is using a plastic tray or on a flat mortarboard, premixed plaster can be mixed in the same way as described previously; however, for plaster mixes that use separate ingredients, these should be dry mixed before adding any water.
For more details on the different types of plaster and when to use them read our Guide to types of Plaster
Use a shovel or small trowel to fold the gypsum and aggregate together, working from the outside inwards to keep the mixture on the mortarboard.
As you would for a cement mortar, form the dry mixture into a mound in the center of your mixing surface and make a small crater in the center, then pour clean, freshwater gently into the hole and fold the plaster inwards,
Use the shovel to keep folding for the outside inwards and when the mixture allows chop through the center with the shovel blade held vertically making sure the dry powders are completely mixed.
As before the plaster should be mixed until smooth and creamy like toothpaste. You can then transfer this onto a plastering hawk and apply it to your walls or ceilings.
Make sure you thoroughly wash down the mixing bed before the plaster has time to dry.
Mixing Filling Plasters
For small repairs, rather than using a large bag of gypsum plaster, Cellulose filler makes a better and quicker job.
Use a small tub or bowl to mix the filler, Like to use a small plastic Ice Cream tub as they are easy to get hold of and clean out after use.
Pour a small amount of cellulose filler into the bowl and using a filling knife or flat-bladed scraper gradually pour in some clean freshwater, be careful with the amount as it doesn’t need very much water.
Fold the powder into the water, gently mix the two components together, stir the mixture until the filler is thick and creamy.
If you do add too much water and the mix is a little too runny, add some more powder filler and keep mixing until it is right. For deep cracks or holes making the mix a little stiff can be helpful as it holds onto the crack better, you can then recover the layer later with a looser batch, once it has dried.
Using Bonding Agents with Plaster
Bonding agents like PVA or SBR are applied to the sub base or brickwork that the plaster is to be skimmed over, they help to control the suction of the background stopping the moisture from being pulled too quickly from the freshly applied plaster
Normally the bonding agents are thinned out using water and then brushed over the surface and allowed to ‘tack-off’.
The plaster is then skimmed over the top of this layer. Plaster should only be applied in layers no thicker than ⅜” (10mm) when skimmed on top of a bonding agent.
Learn more about how to apply plaster in our Guide to Plastering Techniques
The undercoat can be built up in several layers until the required depth is achieved. The surface of each layer is scratched to give a good keying surface for the next layer, you can do this with an old scraper or use special scratch tools to make this a straightforward process.
Each layer should be allowed to set for at least 24 hours before the next coat is applied.
PVA or SBR can be mixed in with plaster or with sand and cement and used on top of a priming coat to help fill cracks and small patches, first clean the surface with a stiff hand brush to remove any dust or loose particles
Then apply a thin water/bonding agent mix to the surface with an old brush. I typically make this 1-part bonding agent to 3 or 4 parts water.
You can then mix plaster or sand and cement with the bonding agent to create a stiff mixture for filling cracks, I suggest using a ratio of approximately 1-part bonding agent 1 part water mixed into the plaster, then apply this filler mixture with a trowel, pressing it well into the cracks
Remember to handle any chemicals carefully, always read the labels, and wear the required protective equipment. Also, make sure you clean up as your tools and brushes scrupulously in clean water
For large projects keep a couple of spare buckets of clean water to hand a regularly rinse off your tools as you work
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This article was written by: Richard Quinton – The DIY Help Desk Owner, Engineer & technical specialist.
Richard is one of the key partners in The DIY Help Desk team. He is a qualified Engineer, writer, and publisher, educated to Master’s level. He is a keen advocate of DIY and home improvements.
Richard enjoys helping others to learn new skills and reach their goals and believes that passing his knowledge and experience on through his writing is an effective way to positively impact the lifestyles and well-being of others on a larger scale.