Is Plaster Of Paris Waterproof?

If you’ve got a crack in your wall or a hole in the ceiling, and you need something that will fill it up and give some protection from water damage, then there are two options.

You could patch it up yourself (and save money) or you could call in an expert who’ll use a material called plaster of paris.

One of the most common questions that I get asked: is Plaster of paris waterproof? I will begin by answering that question before moving on to other things.

Is Plaster Of Paris Waterproof?

Plaster of Paris is NOT waterproof, because it is an extremely porous material when dry. It will absorb water when it gets wet, losing its strength. You need to waterproof plaster of paris for exterior use by filling in pores on the surface using a primer coat.

When moisture seeps into these gaps, the mortar holding them together weakens and eventually loses its bond.

In extreme cases, water may even start to penetrate through and through.

The effect of humidity on plaster of paris is similar to wood, where air and liquid can pass through the fibres causing them to swell and warp. In both instances, the problem usually occurs after prolonged exposure to rain.

But unlike wood, plaster of paris can’t be cut away and fixed back together again once the damage has occurred.

When plaster of paris becomes weakened by moisture, the likelihood of further cracking increases. Once these new cracks appear, they’re likely to spread across the entire structure until it collapses completely.

is plaster of paris waterproof?

What Exactly Is Plaster of Paris?

Plaster of Paris, or POP as it is commonly abbreviated, is a mixture of gypsum powder and water which hardens when mixed together and laid onto the surface where it needs repairing.

It has been used for centuries as a building material and is still widely used today.

Another common question that people ask me – “Henry, is this stuff imported from Paris?”

Why Is It Called Plaster of Paris (POP)?

Let’s break up the name “Plaster of Paris”.

“Plaster” refers to any substance that causes hardened layers to form on top of other materials. So it includes sand, limestone, clay, chalk, lime etc.

And “Paris”, well, simply means Paris, France – where the original formula was developed! This isn’t imported from Paris and is actually very widely available almost anywhere.

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Advantages of Using Plaster Of Paris

The main advantage of using POP is that it is incredibly easy to use. You rarely need special equipment to make use of plaster of paris.

It is readily available at most hardware stores, and its also pretty cheap.

This is the reason why its very popular for making spot fixes or patching up small cracks.

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Drawbacks Of Using Plaster Of Paris

The main drawback of POP is that it is not very durable and therefore not suitable for long-term use. If left untreated over time, it tends to break down and lose its strength. This means that it cannot be used to repair cracks and holes permanently – it would have to be regularly replaced.

This is because plaster of paris breaks down more quickly when exposed to moisture. However, because it is so easy to work with and available at most hardware stores, many people choose to tackle minor repairs themselves without professional help.

What happens if plaster of paris gets wet?

If it gets wet, it will start to dissolve and lose its shape.

It won’t take much water to cause significant damage to plaster of paris. Even just a few drops on the surface can weaken the material enough to allow leaks to occur. If too much water passes through, the cementing agent within the plaster is bound to dissolve.

As soon as this happens, the whole structure starts to disintegrate and loses its integrity.

In order to prevent this happening, you must seal the cracks and holes before they become large enough to leak through.

How To Seal Plaster Of Paris To Protect It From Water Damage?

Fortunately, there are several ways to protect plaster of paris from moisture damage.

These include using products such as mastic, PVA (polyvinyl acetate), urethane acrylate, polyurethane foam or expanding polystyrene beads. Alternatively, you could try covering the area with a non-woven fabric coated with silicone before applying the sealing paste.

These substances should provide adequate protection against moisture penetration for short periods of time.

For example, PVA can remain effective for 12 months while others like mastic can last for upto three years.

However, none of these treatments offer the same durability as plaster itself. It might be worth considering investing in an external plaster board.

What Can I Add to Plaster of Paris to Make it Waterproof?

Option #1: Apply acrylic coating when the POP has dried. To be on the safe side, apply two to three coatings of acrylic to protect against water.

You have to be careful that you do not miss any spot – it just takes a tiny little gap to cause POP to get wet and lose its strength.

Option #2: You can apply any oil-based primer on top of the dried POP. But we have now ventured into the trial-and-error category.

This should in all likeliness help, but your mileage may vary.

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Wrapping Up

I hope this article has satisfied your curiosity about is plaster of paris waterproof.

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Meet The Author: Henry

Hi, I'm Henry. I'm an interior decorator and I love turning people's homes into aesthetically pleasing and functional spaces. I love to take on a good DIY project, play with my daughter and spoil my wife.