What’s the difference between Gypsum and other Types of plaster? Our must-read guide to Plaster and which is best for your project
Plastering is often considered a bit of dark art, with many DIY’ers preferring to employ the skills of a local professional rather than attempt the project themselves, but it’s not really that hard, and with a little knowledge and some practice almost anyone can achieve reasonable results.
Some of the first things to understand are what plaster is and which type is best suited to the job you need to do, once armed with this information the rest is down to practice.
Let’s start a the beginning and look at what plaster is:
Plaster is a mixture of gypsum, water, and often sand, Gypsum is a sulfate mineral, called hydrated calcium sulfate (CaSO4·2H2O), which is crushed into powder and heated. Mixed with water it forms a slurry which is applied by hand to walls and ceilings; it hardens when dry to provide a smooth, decorative finish.
The manufacturing process crushes the block mineral into fine particles, which are then heated in a kiln to 356 degrees Fahrenheit (180 degrees Celsius) this removes most of the moisture from the rock; this results in a fine powder that sets hard when mixed with water.
Modern buildings and domestic properties use plastering on internal surfaces to provide a smooth, stable, and decorative finish for final topcoats, such as painting or wallpapering.
Manufacturers like British Gypsum produce a range of grades of plaster to suit different backgrounds or sub-bases, and the type of finish required.
The trade name for British Gypsum is ‘Thistle’, and can be sourced from various hardware suppliers and stores.
These typically consist of processed gypsum or mixers based on cement lime and sand. Manufacturers use various additives to provide plaster with various properties and characteristics.
Some Plasters are moisture-resistant, whilst others are hardwearing and suitable for high traffic areas
I have researched several sources and pulled together the information below to help provide you with an understanding of which plaster you need to get the best results and provide you with a great finish that should last for many years.
you can also find out more on how long plasters take to dry before they can be painted in our previous post here
Base-coat Vs Top-coat, the 2 grades of plaster
There are two main grades of plasters, Base-coat and Finishing or Top Coat, which are commonly used in today’s building and construction industry, Base-coat is used to provide an even, stable covering this is also called a ’floating’ coat, and is this is applied to bare brickwork as an initial layer to prepare for a final smooth Finishing coat
Base-coat plasters are coarser than finishing coats and often include small particles of lightweight aggregate, and offer good adhesion to block or brickwork, and are used to level out the surface of the brickwork and provide a good, keyed surface for the Finishing plaster.
Basecoats are mixed with water to produce a thick slurry which is then applied to the wall or ceiling by hand, using a steel trowel. Thickness can vary but is usually no greater than ½” (12mm).
Older properties may have a hand-mixed basecoat plaster applied which consists of cement or cement lime mixed with a suitable amount of clean sharp sand and water to produce the same thick slurry as modern gypsum plaster.
Finishing coat plasters consist of fine grade, crushed mineral mixed with water to form a smooth slurry, which is applied by hand using a plastering trowel, on top of the basecoat plaster
This finer grade of plaster allows for a ‘glass-like, smooth top surface to be achieved by hand trowelling and polishing the surface as it hardens.
The Finishing or Skim Coat plaster is usually applied up to 5/64” (2mm) and can have a working time of up to 40 minutes and a setting time of 1 ½ hours.
For a typical 55.0LB (25.0Kg) bag of Gypsum Finishing plaster, you will need approximately 3 Gallons (11.5 liters) of water to achieve the right slurry for application.
This will provide around 107 sqft (10 sqm) coverage at 5/64” (2mm) thickness, take care when mixing as using too much will make the mixture weak and too thin and too little will make applying it to the surface very difficult and it will significantly reduce the working time.
Choosing the right plaster for your DIY project
The majority of plasters used in modern properties are produced from ground or crushed hydrated calcium sulfate which is commonly known as ‘Gypsum’.
Gypsum plasters are normally intended for use on interior surfaces, walls, and ceilings, and are not intended for use outside. Plasters like Render are specially formulated with the right properties and are suitable to be used on permanently damp walls or external surfaces.
Setting times for gypsum plaster are controlled by the use of retarding additives which give each type of plaster a setting time suitable to its purpose
A typical Gypsum Finishing plaster will have a working time of up to 40 minutes and a setting time of 1 ½ hours, it’s always best to check the manufacturer’s instructions and guidelines particularly if you are working in extreme conditions.
Higher ambient temperatures and air humidity will have an effect on the drying times for most plasters and potentially make the plasters bond weaker.
It is also best not to add more water to a batch of mixed plaster, as this will weaken the plaster’s strength, if it starts to harden or ‘Go off’ it is best to discard the remaining amount and mix a fresh batch to work with.
Plaster of Paris
Plaster of Paris is a Gypsum based powder that is quick setting and uses non-retarded gypsum as the base.
When mixed with water, plaster of Paris has a thermal reaction and gives off heat as it sets.
The powder is available in either a pink or white colour and should be mixed with fresh, clean water into a creamy consistency
Plaster of Paris is used to create or repair decorative mouldings and architraves which are normally used to trim edges between walls and ceilings or around ceiling fittings for lights
This grade of plaster is not used for general plastering as it is too fragile and does not trowell well. It is often used to mould small figures and other decorative items
Carlite refers to plasters that are a retarded and premixed with usually a lightweight aggregate which means they weigh less than plasters mixed with sand. Carlite plasters are mixed with fresh, clean water prior to application and are hand trowelled in a similar way to normal gypsum plasters.
The undercoat mix adheres well to most types of background, brick, block, or Drywall plasterboard. Along with its lightweight, which is typically 50 percent of plaster mixed with normal sand, means that Carlite plaster is easy to handle and the application takes less effort than heavier plasters.
Carlite plaster offers some additional thermal insulation when applied thanks to the pre-mixed aggregate. The typical setting time for Carlite is around 2 -2/1/2 hours.
There are 3 different types of Carlite undercoat that are readily available:
Browning – used for solid walls with average suction
Toughcoat – used on a wide range including metal laths
Bonding – used on low suction materials like drywall plasterboard
The different types are mixed with various amounts of ingredients to make them suited to backgrounds with different suction properties and various textures.
Browning Carlite is formulated to be used on solid backgrounds with average suction such as blocks or brickwork, Toughcoat has a much higher resistance to impact once set and can be used on expanded metal as well as brick and blockwork.
Lastly, Bonding undercoat is used where better adhesion is required, on high-density blocks, concrete, or plasterboard for example. Often the base surface is pretreated with a bonding agent to prime the surface and provide good adhesion.
Generally, when applying multiple layers to build a surface, the same product should be used rather than mix different types.
The finishing coat should be applied to the plaster undercoat as soon as it has set and this can usually be applied by hand on top of any of the 3 types of carlite undercoat
Brands Names like – Thistle plasters
No blog post about plaster would be complete without mentioning one brand name, and in the UK ‘Thistle’ is one of the biggest.
The equivalent product in the USA would be USG Diamond veneer, which is not so commonly used in building construction, the preferred method used in the US for finishing internally is to fit sheet drywall which is then lined and filled.
Hardwall is an impact resistant undercoat plaster that inhibits the formation of efflorescence it is suitable for most backgrounds
There are several types of thistle finishing plaster, the most often used are ‘Multi-finish’, and ‘Finishing plaster’ both are intended for internal use and are applied by hand trowelling, Multi finish can be applied on top of a sound and hard undercoat.
Thistle has a specific mix called ’Boardfinish’ which is formulated to be used on top of dry lining or plasterboard surfaces.
British Gypsum also manufactures special renovating plasters for use on walls that suffer from residual dampness, this type of plaster is premixed gypsum which has special additives that help to work well under extreme conditions and can help prevent mould forming.
Thistle finish plaster which contains a fungicide is specially formulated for use with the undercoat this combination is used for general Plastering and offers high impact resistance and higher than normal resistance to effervescence a lightweight cement-based dry coat plaster is available for use as an undercoat after installing a new DPC
There is also ‘ThistlePro Magnetic’ which has high magnetic properties to aid hanging or mounting features like whiteboards, pictures without the use of permanent fixings. And is aimed at Schools, homes, and offices, making walls or surfaces interactive.
Sanded plasters – Traditional materials
Before the advent of modern plasters and plastering techniques, as far back as the Egyptians, different mixtures of lime and sand were employed for undercoats, and for finishes lime was applied directly on top of the undercoat, only being mixed with water.
Traditional lime plastering that was employed during the renaissance and is still used today often uses wet plaster mixed with horse or animal hair added, which is used to bind the material together.
This type of plaster was used by the Italians and other cultures and then finished with great works of art called fresco’s, the most famous being Da Vinci’s The last supper’.
Generally, lime plasters are not as strong or hard-wearing as gypsum or cement-based plasters, however, it is still used today as an additive to which can improve the workability of sand and cement plasters or render
In addition, buildings that are listed or regulated by local authorities because of special status or age often require traditional methods to be used when refurbishment or repairs are required
A cement-based sanded plaster undercoat can also be a requirement for kitchen and bathroom walls which are constructed using timber or metal lathing.
Cement based coatings can also provide good impact resistance to walls where needed such as high traffic areas and offer support to older brickwork.
For general use and expediency single coat plaster can be applied on different background surfaces and worked by hand trowelling and skimmed to a normal finished surface.
As with most types of plaster, this variety is available in 50lb (25 kg) bags, which are ready for mixing with fresh, clean water. Premixed one coat plaster is often sold in smaller 1kg packs or plastic tubs, which can be used for small patches of repairs.
If mixed correctly one coat will be workable for around an hour after that is will start to set and go off,
It is versatile enough to be applied up to 2” (50mm) in thickness at a time or used for a ⅛” (3mm) skim coat.
For small repair jobs like cracks and patches, a 50lb (25kg) bag of plaster is too much and will be wasted, DIY stores or hardware outlets will often have small tubs of premix or ready to mix fillers for just such repairs.
Fillers, just like gypsum plaster and are fine plaster powders that are often reinforced with cellulose resin. Fillers often have additives that prevent shrinkage and can be used to repair deep cracks, small patches, and everything in between.
You can read our guide to repairing cracks for more details and tips
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.