Questions You Should Know about Interior Surface Smoother

Hi friends, Emmett here! Sarah mentioned she gets a lot of questions about drywall finishing, the texture of the walls in our home, and how to achieve those perfectly smooth walls. I’ve been plugging away on our home office renovation and of course- the designer (aka my wife) requested a level 5 drywall finish… which basically means she wants perfectly smooth walls. That’s always her preference, if possible. I thought it would be helpful to share some visuals and tips for achieving that super smooth finish with a quick skim coating tutorial. Click through for some helpful tips, finishing vocab (to pass along to your contractor, if you’re using one), and some tricks for achieving quality craftsmanship.

When is skim coating handy or necessary? If you’re trying to smooth existing drywall, cover wallpaper texture, or are repairing drywall skim coating is your best bet. In our case, the existing drywall in Sarah’s office wasn’t in great shape and had both texture, wallpaper removal lines as well as a two inch wide stripe around the room from removing the chair railing. Half of the sheet rock in the room is new and the other half is existing, so my goal was to skim coat everything to match, resulting in a level 5 finish.

First up… let’s chat about the finishing levels, which also kind of translates to the phases if you’re going for that smooth look. There are 5:

• Level 0 // This means there is no finishing that has been completed. At this level, the sheetrock is just hanging on the wall or ceiling, with nothing done to it. Screws and joints exposed.
• Level 1 //  Level one means that the joint tape has been installed and mudded, but nothing else has been completed.
• Level 2 // This level means that you have skim coated a thin layer of joint compound over the tape and covered the drywall screws. If you’re planning to tile, it’s fine to stop at this level. Many tract homes receive this level in the garage.
• Level 3 // This stage indicates a solid coat of joint compound has been installed to the tape and screws. Walls that will receive a heavy texture (like knockdown), often stop at this level. Level 3 finishing is meant for a highly textured wall outcome.
• Level 4 // This is your classic drywall finish… the standard or norm. Level 4 receives an additional coat of joint compound to cover the tape, seams, screws, and has been sanded in between. This is the most common finish and can be painted or wallpapered nicely.
• Level 5 // As you might have guessed, Level 5 is the gold standard. It’s as smooth as you can get, or the highest finish possible. There are no bumps, no texture, and it’s achieved by multiple skim coat layers that are finished with a good sanding.

If you’re striving for a five finish, this is what I used to achieve that…

Before we get started… the longer the skim coating blade, the flatter your finish will be. For scale, here I am with my 32″ skim coating blade. If your wall is very unlevel or uneven, you’ll probably want to begin with a 24″ blade, and hit the problem areas first before moving to the 32″. Ready to walk through the skim coating process?

First you’ll mix your joint compound with water. I eyeball (rather than measure) the mix, but it should resemble a thick cake batter consistency. Use your mixer attachment on the power drill until it’s smooth and well combined.

Once you get a good consistency, you’re ready to paint it onto the wall. Using the roller with the 3/4″ nap, roll a section of the wall (I work a 32″ wide section about half the wall height at a time to prevent my mud from drying out too quickly). The texture from the nap should look something like this…

Below is a closer look at the texture the roller leaves behind. It looks kind of scary, as we’re going for perfectly smooth walls- but the key is working in thin, layered coats.

Next, grab your skim coating blade and with firm, even pressure, sweep it across the roller texture. The goal is to remove as much excess joint compound as you can. Think of this as smoothing rather than building up a material. Work in sections and think thin layers!

Hold the blade at a sharp angle and work quickly, as the mud dries out quickly. The goal is to work in long even sweeps with the blade, without lifting it from the wall. Many walls will require multiple coats. If this is the case, I try to pull my blade one direction (think up and down) for one coat, and then on the second coat I change directions (side to side). Whichever the direction- just make sure it’s a consistent, fluid motion.

Once you have one section of the wall finished, move onto the next, repeating that same process. You will want to clean your blade in between sections, as the joint compound dries quickly and a clean blade works best for that ultra smooth finish.

Reload your paint tray with joint compound and start on the next section. Using a bucket with a lid will keep the viscosity consistent for longer as you’re smoothing with the blade. I prefer to mix a large batch, which requires less time and cleaning.

Once the walls are totally dry, you can begin to apply additional coats. Sarah’s office walls were pretty smooth to begin with and I ended up installing three layers. One layer per day. Once the wall are done you will then have to use a corner trowel and apply a coat to the corners for a consistent finish. Give the final coat a full day to dry, then you’ll sand the drywall for that ultra smooth finish. In color, it may not look perfectly even, but as long as it’s super smooth, the primer will do the trick to even the color prior to paint. It should look something like this (don’t be alarmed by color differences)…

If you’re looking for a level 5 finish and are using a contractor, it is absolutely something you will need to discuss with them in advance, as it’s a premium finish and not the standard (which is level 4). It really doesn’t use a whole lot more material, however it is very labor intensive. One last thing to note is this tutorial is showing skim coating over a wall that had paint-over-wallpaper. The texture of the wallpaper as well as the seams were visible but it wasn’t in too bad of shape. Skim coating works on textured/popcorn ceilings or any walls that had years of patches and damage to them. This is our perfectly smooth and freshly skim coated wall…

I hope those tips and insight proves to be helpful for your next big project! Would you also like tips on sanding? It’s pretty self explanatory and there are lots of ways and tools to help with sanding… I think that’s just personal preference. We splurged and bought a secondhand drywall sander to make quick work of the project, knowing that 70% of our house still needs drywall work. It hooks into our shop vac which means much less dust (which is exactly how I sold Sarah on that purchase, haha). Hand sanding works just as well though! It just requires more time, with a bit more dust, and a lot more elbow grease.

Let me know if you have any questions in the comment section and I’ll get back with you sometime this week. Thanks for reading and cheering us on with the office reno… it has been a slow go, but we’re cruising along.

Surface roughness is an often-overlooked dimensional aspect of the manufacturing process. While more focus is generally given to the composition of a part and its strength, or to its measured dimensions and tolerances, a surface that is too rough can result in increased friction and premature failure of a part. Going beyond mechanical operation, high purity manufacturing requires smooth surfaces within the processing equipment to avoid contamination or build-up within it. Simply put, the smoother a surface is, the less likely it is that material will stick to it. And of course, the smoother the surface, the easier it is to clean.

How Smooth Is Smooth?

We interact with smooth surfaces throughout our everyday lives. Glass windows are smooth, skin is smooth, and the touchscreen on our mobile devices are smooth. Or are they? Anyone who paid attention in biology class knows that, when viewed up-close skin is not smooth. But touch screens? Those are manufactured with a controlled roughness because too high of a gloss makes the images hard to see. Additionally, the screen would be too slippery to accurately interact with the virtual keyboard. This roughness is measurable in microns (µ), which are .001mm, or micro-inches (µin), which are .0001”. But, while the roughness allows you to interact with the display, it’s also what allows you to leave smudges on the screen.

Different Ways of Measuring Roughness

Roughness itself is a series of microscopic “peaks and valleys” across a surface. This becomes clearer when viewed in cross-section. Surface roughness is calculated measuring the average of surface heights and depths across the surface. This measurement is most commonly shown as “Ra” for “Roughness Average” and that value is used to determine compliance of equipment with various industry standards.  Formally, Ra is described in ASME B46.1 as “the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length.” Ra, Rq (RMS), Rv, Rp, Rz and some other parameters are two-dimensional in nature, being only concerned with ‘up and down’ measurements in a straight line. They do not take into consideration other components of the surface topography such as flaws, errors of form, or waviness (symbolized as Sa, Sq, Sz) that would be measured in a 3D evaluation. Two dimensional roughness measurements are usually taken across any grain that might be present.

For those who are fond of math, the formula to calculate the main height over the entire measured length or area is:

1/n * SUM(ABS[Zi-Zmean] from i = 1 to n

Previously, surface roughness was calculated by “Root Mean Square” or “RMS” which used the same measure of peaks and valleys but utilized a different formula. RMS is sensitive to larger peaks and valleys, where RA is not. RMS, or Rq, will mostly appear on older technical drawings as it has been phased out in favor of RA. Additionally, RMS was typically measured in inches while RA is usually measured in millimeters in most countries other than the USA as most industries now use metric. Many drawings in the USA will show metric measurements with English in parenthesis, i.e., “0.8 (32)”. Also worth noting, the measure of the absolute distance between the highest peak and the lowest valley is shown as Rz.

Ra is measured using a profilometer. This is an instrument with a stylus that travels across the surface and measures the difference in height between the peaks and valleys of the surface profile. ISO standards use the term CLA (Center Line Average), which is interpreted identically to Ra.

Why Does Ra Matter?

As mentioned earlier, a smooth surface makes it harder for the product within the system to stick to the sides of a vessel or piping. Similarly, should free iron or other unwanted material be introduced into the system, there is less likelihood that it will become embedded into the metal and become a source of contamination. With high-purity processes, any contamination can spoil an entire batch of products. While the cost clean and purge a system can quickly add up, there is also the cost of the lost production time to consider as well. Overall, the lower the Ra, the higher-purity production application of the vessel. Not only is it easier to clean, but a smoother finish means that it is easier to empty product. The savings in time between batches can increase the throughput of product, which benefits the bottom line.