How to Prevent Plaster from Cracking When Drying?

If you plan to plaster or frequently do, it is essential to know that plaster is not a flexible material. It does not stretch or adhere to its surroundings very formidably. Instead of adapting (in the form of expanding), the plaster will simply crack. There are techniques you can implement to decrease the potential effects of cracking in your plaster and prevent this blemish from occurring in the future. 

How do you prevent plaster from cracking when drying? The plaster will shrink as it dries, but you can avoid cracks by changing your plaster mixture. Try the following:

  • Use low-cement additives.
  • Use higher-quality sands.
  • Mix more thoroughly.
  • Increase the moisture.

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Many common defects appear in plaster applications, most of which are caused by a poorly mixed batch of plaster. Use this quick read to understand the most common factors ruining your plaster and how to diminish these risks in the future. 

Why Your Plaster Cracks When Drying

Cracking results from a lack of moisture that can cause crazing, plastic shrinkage cracking, or map cracking. You have to understand why your plaster cracks before you can understand how to remedy that imbalance.

The main reasons behind why your plaster cracks when drying include:

  • Sand or grain is poorly mixed – If you don’t have a professional-grade mixer, you risk small particles of sediment being at a chunky texture within your plaster mixture. This is the fastest way to see cracks in your finished product because everything is not only separating; they’re already separate. 
  • There is direct sunlight – Sunlight will oxidize the plaster at a more rapid pace. The heat and UV rays will increase the drying time at a faster pace than the plaster can withstand. 
  • Too much direct wind contact – Drastic aeration and air-circulation will dry your plaster prematurely as well (often in combination with sunlight if outdoors).  
  • The bricks or surrounding material is absorbing moisture – If the product you are adhering to the plaster isn’t dampened beforehand, the moisture can be sucked out of the plaster resulting in cracks. For instance, bricks are a porous material that will suck all the moisture out of the plaster before it can adequately set or adhere.
  • Not using a Masonry Cement – Or another additive that assists in breaking down the sand material. If there is sand that is not very fine, you will require this even more-so to guarantee a fluid and smooth mixture. A product such as this is explicitly used for filling cracks; Adding it beforehand will only cause the binding to occur inside of those cracks in the first place. 
  • Painting Over it Before it Dries – This is another common culprit behind cracked plaster. Do not paint over a surface until you are confident that it is completely dry. If you cut off this air supply, the plaster will break through the paint to gain access to air-flow, causing many cracks that will separate your paint layers as well.  

The Main Types of Plaster Cracks – And How to Prevent Them 

Cracks that appear rapidly are an immediate indication that you’ve done something wrong in the mixing or application of your plaster. The following are the main types of plaster cracks and how to prevent the occurrence of them:

1. Plaster Shrinkage Cracks

This indicates that excessive moisture has been lost in the early hours of application. 

The methods to reduce them are:

  • Add water to the wall before application to saturate the surface first and to minimize brick absorption.
  • Decrease direct contact with sunlight, wind-flow, or any extreme aeration impact. This can be done by starting earlier in the day or the evening, but not the peak-heat of the afternoon.
  • Use thinner sand or particles for the fine material.
  • If the sand is still thick, add a product such as a masonry cement or lime to smooth the mixture.

2. Debonding

This is a common effect of plaster when it starts to separate from the wall. It often means that you didn’t prepare the surface properly to bond with the plaster. 

The methods to reduce them are:

  • Dust the walls before applying the plaster.
  • Add oil or water, something to ensure the moisture isn’t sucked from the bricks, which will only enhance separation and debonding.
  • Use a Cement Slurry or Water Additive as a base coat – before applying the plaster mixture. 

3. Map Cracking and Crazing

These are very similar and will result in very fine, thin cracks spreading out (as the name indicates) like a map. These cracks will be smaller and more detailed than traditional plaster cracks. 

The methods to reduce them are:

  • Using a smaller percentage of concentrated cement content in your plaster mixture
  • Ensuring the sand is fine, thin, and adequately mixed 
  • Don’t allow the drying speed to be too rapid 

4. Popped Cracks (results in the most significant cracks)

These are more like a bubble popping within your plaster mixture. It means that a contaminant particle has entered your mix that the rest of it is separating from. Like oil to water, this expansion can cause a large crevice, bubble, or open crack in your plaster.

The way to reduce them is:

  • Often it is a small particle or organic material that has entered by accident. Minimize these as you can, and if they occur, you can quickly fill the plaster with filler. 

How to Fill a Crack with Sealant 

Plaster is not a perfect medium, and inevitably, it will crack. If a large or hairline crack occurs, you will likely be able to handle this as a DIY project individually. When you have to fill a crack, it is advised that you use a mildew-resistant Sealant, such as acrylic latex caulk

Once you’ve purchased your selected caulk or sealant, the steps you can easily follow are:

  1. Clean out the crack thoroughly and remove any debris, dust, or particles (as you know, this can cause further popped cracks).
  2. Place a sliver of this adhesive within the crack.
  3. Slide your gloved finger over the crack to spread the caulk.
  4. Allow it to dry for the allotted time on the directions, based on the given product you purchase. 
  5. Layer as needed to smooth out the crack.
  6. If you exceed the crack or it protrudes outwards, sand it down. 

It may take a bit of experimentation and sanding or filing down for a smooth finish, but you must allow every layer to dry sufficiently. If a crack does appear, do not panic as it can be easily painted over. 

Final Tips for an Uncracked Plaster

Some last hacks for getting the most out of your plaster application include:

  • Use high-quality paint – Low-quality paint will cause poor adhesion quite rapidly. The products you put within your mixture matter significantly, and so do the products you place on top of this plaster. You should use the same type of paint and not combine different brands, as this can cause separation as well. 
  • Get a professional mixer – To properly mix sand or small particles into your plaster evenly and thoroughly, you will probably need something more adept than a wooden spoon. An example of a quality mixture is The Portable Electric Concrete Plaster Handheld Mixer.
  • Work in small layers – Often, the mistake that people make is lopping on the plaster instead of working in incremental coats. By creating thinner layers, they will:
    • Dry faster
    • Absorb fewer oxygen bubbles for less separation
    • Dry smoother, flatter, and more evenly
    • Reduce cracking

In Conclusion 

The steps you take before applying your plaster will undoubtedly be the most vital for a quality finish. By utilizing these hacks and steps, you will see a positive impact in your future plastering, fewer cracks, and less work for you!

Related Questions

  1. How do you keep shrinking cracks from drying out? Some contractors find that plastic shrinkage cracks can be prevented in hot dry climates by spraying an evaporation retardant on the surface behind the screeding operation and following floating or troweling, as needed, until curing is started.
  2. What causes hairline cracks in plaster? The movement of contraction and expansion can cause hairline cracks in plaster. Bad quality paint can result in bad paint adhesion to the wall, which will lead to cracks over time.
  3. Are cracks in plaster walls normal? Cracks in plaster walls are normal. Due to buildings moving over time it can cause the walls to crack. It will take time for this to happen but is normal.

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