Never use latex paint on a wall that has at any time been painted with an oil base paint; unless of course, you prepare the wall appropriately. I recently did a project where I painted an accent wall in an office building teal. Before painting, the wall appeared to have a somewhat glossy appearance. Upon a closer look, I saw drip marks all over the wall. You did not see them from afar, but they were defiantly there, and they were quite large and all over the wall. Because the wall was white, it hides its imperfections well. The darker your paint is, the more meticulous you have to be because the dark color will show every tiny flaw.
Concerning the wall I was painting, my thought was that perhaps the painter before used bad paint or maybe was a little sloppy. The glossy appearance is not uncommon as you can pick the sheen you want on your wall – flat to high gloss. The paint we buy today is far more superior to the paint we used just a few years back. Typically a primer is already included in the paint, so primer is not always necessary. When to use primer usually plays a role when you choose a new color. For example: if your wall is blue and you want to paint it yellow, and if you skip priming the wall, your finished color will have a green hue to it. Yellow and blue make green. We don’t need as much paint because if the wall is in excellent condition, then one coat is acceptable. I want to note that I don’t care if the paint says you only need to apply one coat. I am a minimum of two coat painter. I want perfection and total consistent coverage.
As the paint dries, it shrinks, and sometimes you will see small areas where the under color is present. I always sand the entire wall with very low grit sandpaper. It is a light sand, and I do it because over time our walls get dirty and things stick to the wall. I want the wall in perfect condition before I begin painting. I patch all the holes and sand the entire wall. Wipe it down with clear, cold water until a white cloth stays white, then I let it dry. After those steps, I run my hand over every inch of the wall to make sure I don’t feel anything but a smooth surface. Yes, I am a perfectionist when it comes to painting. I also sand because it breaks the surface – also known as roughing up. It ensures that the new coat will adhere. It is the same concept as sanding between each layer of polyurethane.
So back to this wall. No matter what roller I used or the attention to detail I gave the wall I could see roller marks once it dried. I am a very experienced painter, and I have never seen this before. I followed all the steps described above, so I was a little confused about why I saw the roller marks. Another odd thing happened after the second coat and short drying time, a place where I had patched the wall and allowed the sheetrock mud to thoroughly dry and then sanded it, just fell off the wall for no reason. The two coats of paint just fell off the wall. Odd. Paint takes a while to dry. Even when the paint appears dry, it has not fully cured. I like to wait at least two days or longer if we have a lot of humidity in the air before hanging pictures or even touching the wall. So I thought perhaps the wall just needed to dry. I painted on a Friday and would check on the wall Tuesday when the office opened back up. To my complete shock, the wall looked the same.
Now when I say roller marks were there, it was minor. My client loved the wall and didn’t notice. The end result is a reflection of my work, and so I went back and painted the wall with a third coat of paint. I got a very expensive and high-quality paint roller, roughed up the wall again, taped everything again. I trimmed out the wall. Meticulously applied another coat making sure I went slow and up and down the wall from top to bottom. The wall needed to dry. It looked good, so I left. My client sent me a picture on Monday and those darn marks where there but in different areas now. Okay, now I was utterly perplexed. I had this gut feeling that would not go away. I have painted a million walls and I knew what I was seeing after plenty of drying time on the teal wall was a result of a compromise in the materials.
It was my 27 years of pharmacy experience and compounding that helped me figure out what was happening. At some point, oil-based paint was used to paint the wall. And what I saw – roller marks – is all a matter of a base and how they separate and suspend (like water, oil, color, and painting latex over oil). If I had not thoroughly sanded down the wall before painting with latex paint, the paint would have peeled right off the wall. So it is not all doom and gloom – minus the roller marks being present. The paint will hold up, and it is unlikely anyone would ever notice anything. What you could see is minor chipping of the paint – no matter how long we allow it to dry, Latex paint over oil paint never fully cures. There were three coats between the oil base and latex, but it is what you cannot see that is the bigger problem.
When I sanded the top layer off the existing paint – which had a sheen to it, the properties of the oil base paint (specifically the oil) is in all the pre-existing layers. You cannot sand off the surface of a wall painted with oil-based paint and remove the oil. That compromises all non-oil based paint layers applied on top of the oil-based application. Lightly sanding latex paint to remove any sheen is okay. The chemical that causes the sheen sands off and what is underneath is almost like flat paint. The gloss is lighter than the base elements and comes to the surface of the paint.
Let’s dive a little deeper in so that I can give some comparison that will make this very easy to understand. If you use the comparison of putting latex on oil base paint to trying to mix oil and glue, then you can conclude that oil always separates. The only thing that does not separate from oil is oil. You can shake the daylights out of it, but it will never entirely and consistently mix. Let that mixture sit for, and the oil will float to the top. And the color will separate and pull to the side of the paint can. Alternatively, you cannot add things to latex paint because it loses its consistency and will get very thin quickly. Even the smallest amount of water will completely ruin the entire can of paint. Water compromises the color that suspended in its base. You can never really get latex/mix shaken enough for it to hold all elements consistency.
If you put sand and water in a bottle and shake it, it will mix pretty good, but the sand quickly settles to the bottom. Sand which is what you are trying to suspend into the water is a heavy particle. The thicker your base, the better it will mix. But the particles you are trying to combine will still begin to settle. If you put sand in oil and shake it, it will stay put longer because oil is thicker and slows it from dropping to the bottom. It will, however, separate eventually. Think of homemade salad dressing with herbs in it. Olive oil and herbs! If you add water to oil-based paint, the water will float back to the surface quickly. Water is lighter than oil. It is very rare for contractors to use oil base paint on walls. They do it is because they can mix it with cheaper oil-based paint and make it go further. Lastly, oil-based paint is less expensive compared to latex.
Pros and cons of oil versus latex paint.
Oil-based paints are less expensive and cover more area than latex paint. Oil base is very durable. It is best used on trim work because we can hit it with vacuum cleaners, knock the door frames when moving furniture, etc. Latex paint is the best option for covering walls. Latex is the most commonly used paint. In the future, the cost to the consumer is lower because the primer is not needed unless there is a color bleed through issue. Having a super durable paint on the wall is not necessary. If a hole gets punched in the drywall, it is easy and inexpensive to repair. Damaged trim (typically made of wood composite wood or synthetic materials) is expensive to fix. If the damage is minor, you could sand it and repaint it. Trim that is damaged typically has to be replaced, and it is costly.
So what am I going to do?
The way to fix the teal wall is to painfully sand the teal paint off, painting the wall with a special tinted primer to cover up the oil base layer and repaint it. I am going to repair the wall because I like to look in the mirror and like the person looking back. It has taken me a lifetime to build my integrity and high work ethic. I wouldn’t compromise that for anything.
See those pesky roller marks! Even when the paint thoroughly dried over four days, the marks were still there.
**** Once I repair the wall I will post a picture of the results.
Disclosure: I am not a licensed professional painter. The comments above are based on my personal experiences and opinion of what works best for me as a DIY painter. My conclusion of what happened during the painting process was confirmed by a Sherman Williams professional.