Skim Coating a Drywall Ceilingby Lorenz Prem
My house has popcorn ceilings all throughout. This look must have been very appealing to someone living in the 60ies, but it's not the look I want. I decided to do the work and upgrade to a modern flat ceiling.
After taking down the popcorn my task was to apply a skim coat across the entire ceiling. Let's take a look at what's involved.
What is a skim coat
A skim coat is a very thin layer of drywall joint compound that covers an entire ceiling or wall. It is smooth and hides all imperfections in the drywall beneath it. A skim coats is the final step necessary to achieve a level 5 drywall finish, the highest level of finish available.
The skim coat only needs to be think enough to cover the drywall. Any thickness in excess of that requirement does not yield any benefits. The thickness of the skim coat will vary slightly due to the inconsistencies in the application. As long as these differences are not discernible with the naked eye they do not diminish the quality of the skim coat.
Choosing joint compound
As mentioned before, a skim coat is done with joint compound rated for this application. Different types of compound excel at different tasks. For example, top-coat joint compound may not adhere to paint well enough to be used for a skim coat, but it's much easier to sand when used as a top coat over other types of joint compound.
A plasterer's trowel is the best tool for applying and smoothing the joint compound away from the edges. Applying the compound with this tools is significantly faster than using a drywall knife. A 12" to 18" trowel is a good choice for the first timer. Professionals sometimes use longer models, which require more dexterity to use.
Trowels are usually paired up with a hawk. The hawk keeps joint compound ready in one hand, while the other is finishing the ceiling with the trowel. You can skip the hawk and apply mud to your trowel straight out of the bucket with a small drywall knife. That frees up the second hand during the application process. Either way, you'll get the job done.
A long drywall knifeworks better than the trowel around the edges of the ceiling. Use a 12" knife to cut-in the edges just enough to get the trowel going. You'll need a mud pan to store the joint compound and to clean the knife regularly.
Being able to reach the ceiling without having to use a ladder is a big advantage. If you are doing the entire house, consider renting stilts. Otherwise you'll have to climb up and down a ladder all day long. Stilts significantly simply the operation. Plus they make for great facebook pictures.
For sanding a pole sander with sanding screens will provide the necessary leverage. For large areas it is more important to go through the different grits of sanding paper. Otherwise you'll tire out too fast. A powered drywall sander such as the Porter-Cable 7800 produces the fastest and cleanest results. These tools really do work.
Bare drywall or other joint compound are the ideal surface for applying a skim coat. All you have to do is make sure the area is mostly dust free.
Painted walls with flat paint can be coated directly. Clean all surfaces thoroughly with TSP and water. While the skim coat will adhere to the flat paint just fine, you are taking a risk that some other layer below the surface will fail. The walls in older homes have been painted dozens of times. Who is to say that that 20 year old layer of primer 5 levels down will hold another 20 years.
Walls with any sort of sheen will have to be either stripped, sanded, or covered with a bonding agent. Don't try to skim coat a surface like this without doing the prep work. The plaster will come off.
As a first-timer you have to prepare to apply at least 2 coats over the entire ceiling, sand once, and fix defects a couple of times. Professionals with years of experience can do better. Speed should not be one of your goals. Even someone who has never worked with drywall can achieve a perfect result with patience and tenacity.
The first coat is the rough coat. It needs to cover the entire ceiling and fill in the big defects in the ceiling. This coat is relatively hard to level, because it is the thickest. Imperfections created by errand trowel strokes will be numerous. Don't let yourself be discouraged. The wall will look rough after the first coat. It's second coat that yields all the results. Let the first coat dry overnight.
Begin the second coat by scraping the blatant imperfections/mistakes you have made in the first coat off the ceiling with a drywall knife. Bigger mistakes, mistakes with pronounced edges, will be easier to scrape off.
When you are done, apply a second coat using the same process you used for the first coat. Apply the compound using trowel strokes perpendicular to the direction you have used for the first coat. This helps with eliminating imperfections.
The second coat will be much easier to apply. The first coat provides a consistent surface the second coat can grab a hold of. Smooth out the surface as best as you can. It is possible to achieve perfection in this coat. Don't be discouraged. Any mistakes you make can be fixed easily.
After the second coat has dried, scrape off any high spots just like you did before. Lightly sand any ridges until they become invisible. Leave low spots. They are easier to fill than sand. At this point you should have mostly smooth ceiling. A few imperfections remain, which we will take care of one by one.
Wear a respirator. Drywall dust can cause long-term diseases.
Filling in Defects #1
We are done applying mud across the entire ceiling. For here until the finish line its all about spot fixes. Using an inspection light or hand-held lamp search for imperfections. Defect will create shadows that should not be present on a defect free surface. Once located, fix the issue by either sanding it out, or filling it in with additional mud. Let your repairs dry.
You will find hundreds of defects at first. They can all be fixed in this stage. Learn from them, adjust your technique in steps 1 and 2, and move on.
Fixing Defects #2
Repeat the previous step. Chances are a few of your repairs will have to be sanded down. Few low spots, if any, should remain. Once the entire surface is smooth, you are done.
Repeat the defect fixing process as often as you have to. Only perfection will do. Don't ruin hours of work by rushing the last step.
Getting this done right is all about tenacity. Don't walk away unless everything is perfect. If it takes 6 sessions of fixing defects, that's what it takes. You'll get it done eventually. You don't want to keep looking up at that one defect you missed after everything is painted. The first coat is particularly daunting. Remember, it's the hardest one.
Using a trowel efficiently is a learned skill. A professional will the 3-4 times more efficient than a beginner. You cannot expect to get good immediately. At first more mud will end up on the floor than it does on the ceiling. But you'll learn. Experiment with what works for you. Here are a few tips from the pros:
- Just get started. There is no reason to be apprehensive about this task.
- Load you trowel in the center. As you push it along the ceiling mud will be pushed to the edges. If you have too much mud on the trowel, the mud will be pushed out from under the trowel and drop to the ground.
- The angle of the trowel determines how mud is left behind or scraped off. Experiment with the angle.
- The pressure you apply to the trowel also influences how much mud is left behind. Higher pressure can also prevent small bubbles.
- Stir the joint compound before applying it. Straight out of the bucket joint compound will contain air pockets that will be hard to flatten out.
- Work in one direction at a time. This helps with applying a consistent coat of compound. Start in a corner and cover the entire ceiling.
- Unless you are smoothing, don't make trowel strokes perpendicular to the direction you are working in. You'll end up creating thicker areas.
- For smoothing, keep the trowel clean. A dirty trowel will apply mud to the ceiling as you drag it along. During the smoothing process you don't want that.
- If the edges of the trowel leaves excessive marks on the surface, chances are the ceiling or wall is not flat in that area. Try running the trowel perpendicular to the direction that you just used.
- Slightly dry compound behaves differently than fresh compound. Certain defects are easier to flatten after the mud has been on the wall for 20 minutes.
- Keep your hands clean. Mud dries much faster on your hands than it does on the wall. The semi-hard mud will eventually make it onto the wall where it'll make getting a smooth finish much harder.
If you have other tips, please let us know in the comments section.
It's a myth that skim coating can only be done by professionals. A determined DIYer can get it done just as well as a professional. Be prepared, however, to spend 3-4 times as much time as a professional.
The inexpensive nature of the joint compound and tools make this one of the highest return-on-investment upgrades you can make. All the value is in the labor.
The only required skill to complete a perfect skim job is tenacity.