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Painting Plaster with Acrylics

Iridescent Bright Gold

On the left, an uncoated hydrostone plaster object with 5% Acrylic Modifier for Plaster. On the right is the same object painted initially with High Flow Acrylics, then Fluid Iridescent Bright Gold (Fine).

Plaster has been decorated with paint since its earliest known usage back in 7500 B.C. Jordan. The rather simple mixture of lime powder, crushed limestone and water was used to achieve smooth walls for interior wall decoration. Ancient Egyptian builders employed calcined (heated) gypsum into their plaster, which improved the functional use. The duel use of form and function is evidenced by their opulent palaces and tombs, including the pyramids of Giza. The term “Plaster of Paris” originates from 15th century Montmartre, France – the source for its prized hemihydrated calcium sulfate. Of course other sources are now known, but this specific vein of gypsum was proven to be an extremely important component for mold-making and casting and the name is now synonymous with plaster. Traditionally shellac is used as a plaster sealer, but this article focuses on using waterborne acrylics for both sealing and painting.

ATE

Airbrush Transparent Extender is used to seal the surface of the cast plaster. The darker area has three thin layers applied.

Before painting plaster, it is important to allow the material to become fully dry. This process may take up to a month, so follow the plaster manufacturer’s recommendations on curing time. Ultimately, the plaster needs to be very dry in order for the initial priming layers to properly bond to the plaster surface. Drying rooms or low temperature ovens expedite the drying rate for objects.

Acrylics are alkaline in the wet state and adhere well to plaster. However, the surface of cast plaster is usually smooth and thicker paints often cannot penetrate deep enough to create a strong bond. Therefore, thin products are better suited as the initial coats. Uncoated plaster is extremely absorbent and allows thin products to penetrate deeply and seal the surface, thus paving the way for the next layer. This is the key to painting with acrylics onto plaster. Allow time for each layer to sufficiently dry. An indicator of this is the plaster darkens while wet, and then lightens as it dries. As each coat continues sealing the surface, the plaster remains a bit darker . Once the surface is sealed, thicker products such as Fluid Acrylics or Heavy Body Acrylics can be used.

GOLDEN High Flow Acrylics may be applied directly to unsealed plaster. These paints readily absorb into the plaster. The negative side effect is akin to staining raw wood; the color tends to be somewhat blotchy and uneven.

Lab Test

A Brookfield CP3 Texture Analyzer is preparing to test the durability of a plaster disc tinted with Fluid Indian Yellow Hue.

Woodworkers remedy this with a pre-stain conditioner. A ready to use, suitable conditioner for plaster is GOLDEN Airbrush Transparent Extender. This thin acrylic medium is applied directly to the plaster surface. Two or more coats create a less absorbent and more uniform paint layer. Certainly other products may be used (GAC 800 is an excellent coating for chalky surfaces), provided they are sufficiently thin enough to readily absorb into the plaster. High Flow Acrylics were compared to Fluid Acrylics on bare plaster and both absorbed readily with excellent adhesion. These paints are thin enough to not cover up casting details. GOLDEN Heavy Body Acrylics can be used on a properly sealed surface, but they will retain more brush and tool marks.

Acrylic paint may also be added into the plaster slurry to tint the cast object. This is a useful coloring method because, unlike surface painted plaster, chipped areas will not be as noticeable. Adding acrylic paints or medium may also improve the overall plaster strength because they form a network within the plaster. The concept of adding acrylic to strengthen plaster is not a new idea. GOLDEN Custom Product “Acrylic Modifier for Plaster” was developed around 1990 to improve the overall strength and chip-resistance of cast objects by integrating a matrix of acrylic polymers within the cast object. The liquid additive is mixed with water prior to adding the powdered plaster, usually between 3-10% of the weight of plaster used.  At a 10% level, our limited testing revealed that the Acrylic Modifier for Plaster increases the amount of force required to break ordinary Plaster of Paris by 50% and increases the amount of force required to break USG’s Hydrostone by 25% . Hydrostone is a stronger and more durable plaster to begin with, so this increase is noteworthy. Adding acrylic into the plaster also reduces absorbency, thereby reducing the number of sealing coats required prior to painting. However, acrylic additions will also slow the curing time down, which can be an issue for some applications.

AMP

GOLDEN Acrylic Modifier for Plaster and water ready to be added into DAP Plaster of Paris.

Once the plaster objects have been painted, they may be top-coated with a variety of materials, including GOLDEN Polymer Varnish, MSA Varnish or Archival Spray Varnish for decorative and artistic needs. Commercial topcoats should be used for more demanding functional requirements. Exterior use demands the use of waterproof coatings such as two-part automotive urethanes.

As always, if you have any questions about this application, please contact the Material Specialists in our Technical Support Department.

8 Responses to Painting Plaster with Acrylics

  1. Hugh S. Myers May 17, 2016 at 12:08 pm #

    Excellent article! I’m now surprisingly (well not too) inclined to begin work on plaster. I’m thinking of plaster sheets formed by pouring a 1/2 inch or so in a rectangular mold. The increase in strength may make this a practical idea—time and experience will tell. I’ll start small and work up and at some point add plastic screening at a mid point or perhaps sculpture grade plaster bandaging. Many possibilities come to mind, even polychrome sculpture beyond bas-relief.

  2. Yamin November 8, 2016 at 6:38 pm #

    Thanks for the great information! I have another question: Can I apply GOLDEN Polymer Varnish, MSA Varnish or Archival Spray Varnish directly to plaster as sealant if the plaster is not primed or painted first? Thank you!

    • Michael Townsend December 13, 2016 at 3:16 pm #

      Hello Yamin,
      Thank you for contacting us with your questions. The varnishes may be applied directly, but they will readily absorb into the plaster surface. With gloss products this isn’t a big concern because they help seal the surface for subsequent coats, but satin and matte varnishes absorb in but leave the matting solids on the surface. In turn, this creates a frosted appearance (as seen in this video: https://www.youtube.com/watch?v=4hvobUjgPdQ ). On unpainted plaster you may not notice it, but it can obscure colors a bit. ALso realize that absorbed varnish layers are not removable.

  3. Jemma November 20, 2016 at 8:17 am #

    Hugh, that last idea sounds cool.
    Thanks for the article Michael. I gave my flat surface plaster piece a light spray of varnish and did image transfer on it with a medium but it rubbed off more than another method I’d tried.

  4. Daniel February 19, 2017 at 7:52 am #

    Great article!
    I want to join two fairly large hemispheres (40cm) together and am considering a sealant to seal the plaster before I apply any resins. Is my reasoning sound or am I missing a step?

    • Michael Townsend February 20, 2017 at 7:40 am #

      Thank you Daniel.
      I have never tried to join two casts together but it seems as if it might be better to not pre-seal the pieces to better allow the resin to penetrate into the plaster. I think it would be wise to do a test to see which method has better adhesion using “strips” of cast plaster that resemble the rim edges. What type of sealant are you investigating?
      – Mike Townsend

  5. Aubreigh April 12, 2017 at 6:10 pm #

    I was wondering what kind of outcome I would have if I mixed plaster of paris with the paint and layered it like one would with oil paint; for a textured look on a wood panel. What are your thoughts?

    • Michael Townsend April 25, 2017 at 8:50 am #

      Hello Aubreigh,
      Thank you for your comments. I do not forsee any issues with what you are describing, but I would greatly suggest making test panels to assure you have proper adhesion between layers and there are no signs of cracking in the dried applications. – Mike Townsend

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