If you’re a crafty person, you’ve probably come across Plaster of Paris at least once in your life. Though there are several different types of Plaster of Paris, they all share the commonality of being porous.
For this reason, it’s often recommended to seal any project that you make with Plaster of Paris to maintain its integrity.
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Plaster of Paris – A Quick Breakdown
Plaster of Paris is a powder that quick-sets when mixed with water. The most common type of powder used is gypsum (calcium sulfate), but there are also Lime and Cement varieties that function in much the same way.
Its moniker comes from the abundance of gypsum powder found around Paris in the 1300s. It was commonly used to build statues and make repairs on public buildings.
It’s been used for multiple purposes outside of the building, such as immobilizing casted limbs, fireproofing certain chalked material, and of course arts & crafts!
The Importance of Waterproofing
Even though Plaster of Paris can be set fairly quick, it’s also prone to deterioration if exposed to the elements – especially moisture. Plaster of Paris is not waterproof on its own.
If allowed to get too wet, the bond becomes weaker, and the material can actually warp or sag. A seal of some sort applied on top of a finished product will not only prevent moisture from ruining its integrity of it but will also provide a smooth base for paints or other finishes if you so choose.
Think of Plaster of Paris as the drywall in your house. It serves as a wonderful base and gives structural integrity, but it needs to be sealed (and painted) in order to provide you with longevity.
How to Seal Plaster Sculpture/Objects
There are multiple sealants you can use for Plaster of Paris. They can include polyurethane, acrylic, shellac, or epoxy sealant.
An oil-based polyurethane sealant is fairly hassle-free and quick drying. Applying it via a spray can makes the application quick and easy. However, a regular paintbrush will work just as well.
Polyurethane will fill in most of the pores in the plaster, though a yearly application for outdoor objects is recommended. It also dries to a glossy finish, so if you don’t plan on painting afterwards, this type of sealant is a great option to add shine to your finished piece.
Acrylic paint is especially useful if you are planning on painting your piece after sealing it because it can allow you to combine both steps into one easy process.
Acrylic paint dries glossy and is well-known for being rather impermeable to water and other elements. You can opt for a primer that works with your brand of paint if you so choose, but honestly, a couple of layers of acrylic paint will help to seal the plaster and provide your sculpture with the intended color.
Shellac paint/primer functions in a similar way as polyurethane. The biggest difference is that shellac can cast a slight yellow hue once dried. But on the plus side, it is easier to paint over a shellac spray than it is a polyurethane spray because shellac can function as a primer as well.
Probably the most durable among all the sealants, epoxy resin will provide a hard “shell” finish on any object. Drawbacks to epoxy resin are that it is expensive and requires the correct ratio of resin to hardener in order to work well.
After application, it can be sanded, primed, and painted just like any other sealant. Epoxy resin will provide the most waterproofing abilities to any Plaster of Paris craft.
Can You Paint Plaster of Paris
Yes – with the correct preparations. Water and oil-based paints will soak into the porous areas of the statue and will require multiple coats.
Acrylic paints are the top choice for painting any plaster statue because their chemical composition creates a slight “seal” once they are dry as well as a smooth, glossy surface.
If possible, it’s recommended you use some sort of primer before applying any paint to your plaster piece. This not only adds an extra layer of sealing properties but ensures that any paint you use goes on evenly.
To make sure your project turns out just as you wish with no cosmetic imperfections, take the time to follow the directions for each layer of sealant.
Allow to Dry
Before sealing and painting your Plaster of Paris project, make sure you allow it ample time to completely set and dry.
Most formulations are dry to the touch within an hour and completely set within 12 hours. After that time frame, you can apply any sealant, paint, or primer you want.
Applying anything before it’s set could cause moisture from the sealant and paint to be pulled into the plaster and cause it to become misshapen.
If you are using a mold, chances are there won’t be many edges to sand and smooth out. But if you freehand your project, you should sand down any bumps and unevenness before you apply any waterproofing compound.
Waterproofing & Sealing
Once your object is smoothed to your liking, apply your sealant (or paint) of choice, allowing generous time between each coat to prevent clumping. You can lightly sand or buff any accumulated areas but try not to overdo it, or else you may end up filling down portions of your statue.
After you are satisfied with the sealant and it’s completely dry, you can apply your paint. Or, if you opted for acrylic paint as your sealant, give the project one final smoothing over, and if you wish to, you can apply a final coat of gloss on top to finish it off.
Any item that is kept outside in the rain and sunlight will require more upkeep. Touch up paint and sealant at least once a year to maintain the integrity of your piece.
Items kept inside shouldn’t need touching up at all, but if they do, it should be light spot repair.