Plaster of Paris vs. Joint Compound – What’s the Difference?

Plaster of Paris and Joint Compound are two common materials used by homeowners for patching up holes and cracks in walls.

It’s a pretty similar material, but there are some key differences between the two.

In this blog post, we’ll discuss both these products in detail, the differences between the products, along with the pros and cons of each material so that you can decide which is right for your next project.

What is Plaster of Paris?

Plaster of Paris (POP) is a white powdery substance obtained by calcining gypsum.

To make POP, gypsum is heated at around 150 degrees centigrade.

The resultant product is a non-toxic, fireproof material because of its low thermal conductivity.

Since the product was extensively mined from Montmartre in the Paris district, hence its name.

The Plaster of Paris was first made and used in construction about 9000 years ago by ancient Egyptian, Greek, and Roman civilizations.

It has been used extensively in the construction industry since then because of its versatility and low cost.

The main component of the Plaster of Paris is calcium sulfate dehydrate. When mixed with water, it forms a paste that dries to form a hard and strong material.

It is currently used as a base material in the production of construction materials like mortar, tiles, and bricks. You can also use it to repair or patch the walls before painting.

POP is also used for making artworks, sculptures, castings, false ceilings, Chef hats, and casts for broken bones.

Plaster of Paris is also used in dentistry and medicine for making dental impressions and casts for broken bones.

What is Joint Compound?

Also known as mud, gypsum board compound, or simply joint cement, a joint compound is a construction material used to finish interior drywall joints and corners.

It is composed of a base material, usually Portland cement, limestone, and gypsum.

The base material is combined with water to form a paste. The paste is then applied to the drywall joint with a putty knife or trowel and allowed to dry.

Once it dries, the joint compound forms a hard, durable surface that can be sanded, painted, or textured.

Joint compound is available in powder or premixed form. A premixed joint compound is convenient because it does not require mixing with water.

It is also available in a variety of colors to match the color of the drywall.

A joint compound is typically used for completing drywall projects. It can be used to fill gaps, cracks, and holes in the drywall.

Once it dries, the joint compound can also be used to sand, paint, or texture the surface.

So, what’s the difference between Plaster of Paris and a joint compound? Let’s take a look…

Plaster of Paris and Joint Compound Differences

Manufacturers generally use cross-linking chemicals to bind the mixture in gypsum plaster.

Without these hardening chemicals, the gypsum paste is called a drywall joint compound.

But when the chemicals are added, the mixture becomes a patching compound which is called plaster of Paris or hot mud.

The main difference between the two materials is that Plaster of Paris is primarily used in construction while joint compound is mainly used for finishing drywall joints.

The plaster of Paris is made from calcium sulfate dehydrate (calcium sulfate hemihydrate), while the joint compound is made from a combination of Portland cement, limestone, and gypsum.

Joint compound is also available in powder or premixed form, and it comes in a variety of colors to use on walls.

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For What to Use Plaster of Paris on Walls?

While the joint compound is preferred for patching walls, people also love using POP on walls due to its versatility.

Let’s take a look at different ways you can use Plaster of Paris on walls differently…

1- Repair Work

If you need to repair a hole or crack in your wall, then Plaster of Paris is the ideal material.

All you need to do is mix the Plaster of Paris with water to form a paste.

Then, apply the paste to the hole or crack using a putty knife or trowel. Once it dries, sand the surface smoothly and then paint over it.

2- Adding Textures       

If you want to add texture to your walls, then Plaster of Paris is the perfect material.

With POP, you can create beautiful wall designs & decorations that range from simple to complex.

Mix the plaster with water to form a paste to use for texture. Then, apply the paste to the wall using a putty knife or trowel.

Once it dries, sand the surface smoothly with sandpaper and then paint over it with your color choice.

3- Strengthening the Drywall

Plaster of Paris can also be used to strengthen cracked or otherwise damaged drywall.

In order to use Plaster of Paris for strengthening walls, the wall must be cleaned, and all loose debris must be removed.

Using a putty knife, the plaster can then be applied directly to the wall. Once the plaster has dried, it can be sanded and painted to match the surrounding area.

With a little effort, Plaster of Paris can be used to repair and strengthen walls in both homes and businesses.

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Tips and Steps for Working with Plaster of Paris

Working with Plaster of Paris can be tricky since it hardens and sets quickly. But with a little practice and patience, it can be easily mastered.

Here are a few tips for working with the Plaster of Paris:

Step 1 – Clean the Area

Start by cleaning the area where the plaster will be applied to make the surface free of dirt and grime.

Then use a damp rag or a brush and some mild cleaning solution.

This will help the plaster to adhere better and will also make it easier to remove any excess plaster.

Step 2 – Mix Plaster of Paris

Mix 1 pound (0.45 kg) of plaster of Paris with 6 ounces of lukewarm water in a pail or other large container until it forms a smooth paste.

Make sure to thoroughly mix it before resting for about one minute.

Add some water until you get the consistency you desire, and make sure that you only prepare what you’ll need since it hardens quickly.

Step 3 – Moisten the Area and Apply POP

Wipe the surface to be plastered with a damp sponge or towel, as this will help the wet Plaster of Paris to stay put.

Now using a putty knife, fill up the gaps or cracks in the wall with plaster and smoothen them with the rest of the wall surface.

Remember, if the area you need to repair is large and deep than ½ inch, fill the hole partially and allow the plaster to dry for a couple of hours. Then apply another layer of plaster.

Step 4 – Let the Plaster Dry and Then Sand It

Let the plaster dry completely, which usually takes about 24 hours.

After the plaster has dried, use fine-grit sandpaper to smooth out any roughness on the surface.

Step 5 – Paint Over with Latex Water-Based Paints

A high-quality latex interior paint is what you’ll need to paint over the Plaster of Paris, as this type of paint will adhere well and will also provide a smooth finish.

Make sure that the plaster is completely dry before painting over it.

Use a bristle paintbrush to get your repaired walls and ceilings painted. After the paint has dried, your walls will look new and improved.

Do You Need to Apply Primer On Plaster of Paris Before Painting?

No, you do not essentially need to prime Plaster of Paris before painting over it.

However, if the plaster is very absorbent, you may want to prime it with a stain-blocking primer before painting.

Kilz Premium High-Hide Stain Blocking Interior/Exterior Latex Primer is one of the best to use as it’s versatile. It can be applied on a variety of different surfaces very easily.

Applying this primer will help prevent the paint from being absorbed into the plaster and provide a better surface for the paint to adhere to.

If you don’t like Kilz, there are also the primers from Zinsser and Rust-Oleum that you can try.

Related FAQs

Do You Need to Seal Plaster of Pairs?

No, the Plaster of Paris on indoor walls does not need to be sealed or waterproofed.

However, if you use it outdoors or in an area that is likely to get wet, such as a bathroom, you will need to apply a waterproof sealer to the plaster before painting over it.

What to Use to Seal and Waterproof Plaster of Pairs?

Several different sealers can be used to waterproof Plaster of Paris, but the best one to use is a shellac-based sealer.

Marine-grade epoxy resin also makes an excellent sealer for Plaster of Paris and can be applied similarly to shellac-based sealers.

These sealers will provide a long-lasting waterproof barrier that will protect the plaster from moisture and keep it looking good for years to come.

Can You Varnish Plaster Of Paris on Artworks and Castings?

A clear acrylic varnish can be applied once the plaster is fully dry for artwork and castings made from various plasters.

This will help to protect the plaster from dust, dirt, and other environmental pollutants.

It is important to note that some plasters, such as the Plaster of Paris, can turn yellow over time when exposed to light.

To prevent this, applying a UV-resistant varnish or sealer to these types of plaster artwork and castings is best.

The Bottom Line

Plaster of Paris is a non-toxic, versatile material that can be used for various purposes around the home.

With a bit of creativity, you can use it to repair holes and cracks, add texture to your walls or get them decorated as desired.

Just remember to take your time, mix it thoroughly, and apply it evenly.