Skimming Secrets: Mastering The Art Of Plaster Of Paris Skim Coats

how to skim coat plaster of paris

Skim coating is a process that involves applying a thin layer of joint compound or plaster to walls to create a smooth finish. While it can be a messy and tiring process, it is a cost-effective way to achieve smooth walls. Before beginning the skim coating process, it is important to prepare the space by covering floors, furniture, and appliances, as well as sealing off the room to contain dust. Additionally, removing baseboards and outlet covers is recommended to simplify the process and protect these elements.

For small areas, ready-mix joint compound is a good option, as it costs around $15 for a 5-gallon bucket. However, for larger areas, powder joint compound may be more cost-effective, as it provides about four times the coverage. When preparing the joint compound, it is crucial to mix it to a consistency similar to thick pancake batter, ensuring it is thin enough to smooth onto the wall without dripping. A mixing attachment for a drill can be useful for this step.

The application process involves working in small sections, using a thick nap paint roller to apply a generous amount of mud to the wall. After rolling the mud onto a section, a Magic Trowel or a regular drywall trowel can be used to smooth it out. It is important to alternate the direction of each consecutive coat and maintain a wet edge by lightly misting the previous section before starting a new one.

While skim coating can be challenging and requires practice, it is a rewarding process that delivers smooth walls.

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Use joint compound for the skim coat

To skim-coat plaster of Paris, you can use joint compound. Here's a step-by-step guide:

Prepare the Surface

Before applying the joint compound, ensure that the plaster of Paris surface is clean and free of any bond breakers. Wash the surface with a suitable cleaner and abrade it to create a better bond. If there is any remaining wallpaper paste residue, scrape or sand it off. In addition, if there are cracks or holes in the plaster, repair them with a suitable filler or patch before proceeding.

Mix the Joint Compound

For the skim coat, you will need to dilute the joint compound to a suitable consistency. Follow the manufacturer's instructions for mixing the joint compound, adding water gradually until you achieve a spreadable, cake-like consistency. You can also use a pre-mixed joint compound to speed up the process.

Apply the Skim Coat

Once the joint compound is mixed, you can apply it by hand, with a sprayer, or using a roller. Ensure that you only mix and apply as much as you can work with before it sets, especially if you're using a fast-setting compound. Smooth out the compound with a trowel or a drywall knife, creating an even and uniform surface.

Allow to Dry and Sand if Needed

After applying the skim coat, allow it to dry completely. Depending on the type of joint compound and the thickness of the coat, drying times may vary. Once dry, lightly sand the surface if needed to achieve a smoother finish. Remove any dust with a damp cloth or vacuum before proceeding.

Apply a Second Coat if Necessary

Depending on the desired finish and the condition of the underlying surface, you may need to apply a second coat of the joint compound. This is especially true if you are filling in imperfections or creating a Level 5 finish. Repeat the process, ensuring that each coat is thin and even.

Prime and Paint

Before painting, prime the skim-coated surface with an appropriate primer. An alkyd (oil-based) primer is recommended as it soaks into the joint compound and provides excellent adhesion for the paint. Follow the manufacturer's instructions for drying times, and then proceed with your chosen paint.

Using joint compound as a skim coat on plaster of Paris requires some skill and preparation, but it can create a smooth and uniform surface, making it an excellent option for renovating and refreshing old walls.

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Plaster of Paris is a nightmare unless you have skills and/or time

Plaster of Paris is a material that requires a lot of skill and time to work with effectively. It sets and dries extremely fast, which can make it challenging to use for repairs and skim coating. If you don't work with it quickly and efficiently, it can lead to a messy and frustrating experience.

When using Plaster of Paris for skim coating, it is crucial to mix small amounts at a time as it starts to set within minutes. This can be a challenging process, especially for beginners or those with limited experience. The fast-setting nature of Plaster of Paris can result in trowel marks and an uneven finish if not applied and smoothed out promptly.

Additionally, Plaster of Paris can be difficult to sand, and creating a smooth finish may require multiple coats. It is important to work in small sections and ensure that the mixture is thin enough to apply smoothly without dripping. The process can be physically tiring and time-consuming, especially for larger areas.

To achieve satisfactory results with Plaster of Paris, one needs to have the necessary skills and be prepared for the labour-intensive process. It is essential to be efficient, work quickly, and have a good understanding of the material's properties. For those without the required skills or time, alternative materials, such as joint compound, may be more suitable for skim coating.

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Prepare the joint compound

The first step in preparing the joint compound is to ensure that the surface is clean. The leading cause of failure in skim coating is surface contamination. Dust, smoke, and oil can cause issues, so it is important to use a cleaner like Trisodium phosphate (TSP) and gloves. After cleaning, use a wide knife and topping mud to skim the surface.

For a light skim coat, you can use a topping mud or topping compound, or add water to a standard joint compound or mud. If you are using plaster of Paris, be aware that it sets up very quickly, so it is not suitable for most repair applications. It is best suited for plaster casts, moulds, and pattern making.

If you are skim coating over plaster, it is recommended to use a bonding agent to improve adhesion. Products like Plasterweld or a PVA glue can be used as bonding agents and mixed into the joint compound. An alternative method is to coat the surface with an alkyd (oil-based) primer before applying the joint compound. This allows the primer to soak into the compound and achieve excellent adhesion.

When mixing the joint compound, use powder form and mix it with water. This setting-type compound cures faster than pre-mixed compounds. For larger repairs, you can use products like Durabond or hardware cloth as a lath, and then use pre-mixed compound for the top coating.

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Apply joint compound to the wall

To apply joint compound to a wall, follow these steps:

Prepare the Wall

Before applying joint compound, ensure the wall is clean and free of any bond breakers, such as wallpaper paste residue. Wash the wall with a non-sudsing cleaner and a medium-coarse pad, then finish with a clean water rinse. Alternatively, you can use a bonding agent as a primer to improve adhesion.

Apply a First Coat of Joint Compound

If your wall has pronounced gaps or seams, consider applying a small amount of joint compound into these areas first. Use the edge of a drywall knife to scoop and work the compound into the gap, then smooth the area with the knife blade so it's even with the wall surface.

Cut and Apply Drywall Tape

Measure the length of tape needed to cover the seam and cut a section of drywall tape with a knife. Gently but firmly press the tape onto the wall over the seam, ensuring it adheres.

Apply Joint Compound Over the Tape

Using a drywall knife, apply the joint compound in sections, beginning at the top of the taped seam. Feather the compound over the tape, making light strokes that move downward and to each side in a steady manner. Continue until the entire seam and tape are covered.

Allow the Joint Compound to Dry and Sand

Depending on the brand, the joint compound can take up to 12 hours to dry. Once dry, lightly sand the area with sandpaper and brush away any residue. If the tape is exposed after sanding, apply a second coat of compound, let it dry, and sand again until the wall is smooth.

Additional Tips

  • For inside corners, apply a thin coat of mud to both sides of the corner, then cut, fold, and fit a strip of pre-creased paper tape. Smooth the tape carefully with a taping knife or an inside-corner taping tool.
  • For outside corners, use preformed tape or attach corners with adhesive or nails, then smooth mud over the corners with long vertical strokes.
  • For butt joints (where un-tapered ends of drywall meet), apply mud and tape in the same way as beveled joints, using only enough mud to fill the joint and bed the tape.
  • For large gaps or crushed drywall corners, use powdered drywall mud, also called "setting mud" or "hot mud," which hardens quickly and tends to shrink less than premixed mud.
  • If using mesh tape, apply it over seams and then apply mud on top, allowing an adequate amount to seep through the mesh.
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Skim the walls with a Magic Trowel

Skim coating is a great way to smoothen and repair walls. Here is a detailed guide on how to skim the walls with a Magic Trowel:

Preparation

Before you begin, ensure you have the right tools and materials. You will need a Magic Trowel, which comes in various sizes, ranging from 12 inches to 26 inches. Choose a size that suits the scale of your project and the size of your walls. You will also need joint compound or plaster of Paris, depending on your preference and the condition of your walls. Other materials to have on hand include a bonding agent, such as Plaster-Weld or PVA glue, water, a bucket, and a trowel.

Step 1: Prepare the Wall

Start by cutting away any warped or damaged areas of the finished coat, exposing the brown coat underneath. Clean the surface thoroughly, removing any wallpaper paste or other residue. You can use a non-sudsing cleaner and a medium-coarse pad for this step. Rinse the wall with clean water and a sponge afterward. If there is paint on the wall, ensure it is an alkyd (oil-based) primer, as latex or PVA primers may not provide a strong bond for the compound.

Step 2: Apply the Bonding Agent

Once the wall is prepared, brush on a thin coat of the bonding agent. This step will help ensure that the joint compound or plaster of Paris adheres properly to the wall. Follow the instructions on your chosen bonding agent for proper usage.

Step 3: Mix the Compound

If you are using plaster of Paris, be mindful that it sets very quickly, so you will need to work fast. Mix small amounts at a time, following the instructions on the packaging. If using joint compound, you can use the setting-type compound, which comes in powder form and needs to be mixed with water. This type of compound cures faster than pre-mixed options.

Step 4: Apply the First Skim Coat

Using your Magic Trowel, apply the compound or plaster of Paris to the wall in thin, even coats. Work in small sections at a time, as the material will start to set within minutes. If you are using plaster of Paris, you can add a capful of white vinegar to retard the setting time slightly.

Step 5: Smooth and Finish

Once the compound or plaster starts to set, use a wet trowel to smooth out any trowel lines and create a uniform surface. You can also use the Magic Trowel to finish drywall, knock down textures, and create a smooth, professional look.

Tips:

  • For larger areas or walls with significant damage, consider using a drywall compound, as it will be easier to sand smooth afterward.
  • Always work in thin coats, as they will dry faster and be easier to apply than heavy coats.
  • If you are new to skim coating, opt for a joint compound with a longer working time, such as the 45-minute mud.
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Frequently asked questions

Yes, but it's not recommended unless you have the skills and/or time. Plaster of Paris sets very quickly, so small batches should be mixed at a time.

All-purpose joint compound is a good alternative. It's easier to work with, and you can add a bonding agent to improve adhesion.

Ensure your walls are clean and free of any bond breakers, such as wallpaper paste residue. You may need to rough up the surface or use a bonding agent for better adhesion.

You will need a paint tray, mud pan, or bucket; a mixing attachment for your drill; a thick-nap paint roller; a spray bottle with water; and drop cloths for the floor.

Mix the joint compound to a thin, pancake batter consistency. Work in small sections, applying the compound with the roller and then smoothing it with a trowel. Alternate directions with each coat, and keep a wet edge by misting the previous section before starting a new one.

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