Bumpy or blistered walls are unsightly. They can quickly spread and break when unresolved, leaving paint gaps on your walls. You may keep repainting your walls, but if you don’t fix the root cause, it will happen over and over again.
Bumpy or blistering walls occur when paint loses adherence to the underlying surface (substrate) or base coat. This may result from high humidity levels or hot weather. Other reasons include applying paint on a damp, dirty, or porous surface and painting an overly thick layer of paint.
The rest of this article covers causes and remedies for bumpy walls. I’ll explain the common reasons for this problem and suggest easy fixes to help you get (and keep!) your walls smooth.
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Why Your Walls Look Bumpy
When you notice bumps or blisters on your walls, it’s usually because the paint is lifting off the wall or base coat in those spots. It may be due to environmental conditions such as excess heat, humidity, or poor paint application procedures.
Blisters may appear on your wall soon after a fresh coat of paint is applied or after a while. In some cases, there is little you can do to prevent blistering. However, most of the causes are easily preventable.
Let’s examine some possible causes for bumpy walls in greater detail.
Painting Over a Dirty Surface
Painting over a dirty surface prevents the paint from properly adhering. This is because grime and other dirt particles create a barrier between the paint and the wall, and the paint sticks to them rather than the intended wall, causing bumps.
Painting on a dirty wall may cause you to lose out on your paint’s warranty because it contradicts the proper painting procedure.
Painting Over a Damp Wall
You may thoroughly clean your wall before painting, but if you paint it damp, you will have to deal with a bumpy wall.
Your walls may dampen from any of the following reasons:
- Washing them
- High humidity levels
- Plumbing leaks
- Rain (common for outside walls)
Untreated wood end joints can also allow water to seep through already-painted walls.
Paint doesn’t adhere properly to damp walls, which may be why you have bumpy walls.
Painting Over Porous Walls
Drywall and concrete are some examples of porous wall surfaces.
Porous surfaces have small cavities where air bubbles are trapped during painting. If a primer is not applied before painting, the trapped air can form pockets underneath the paint, causing blisters.
These can worsen in hot weather when the air within the blister expands and causes it to burst open.
Painting your walls when the temperatures are too high will cause the outer layer of the paint to dry much faster than the inner layer, preventing it from properly attaching to the wall. This leads to bumpy spots or blistering.
Latex or water-based paints are more susceptible to blistering in high heat than their oil-based counterparts.
Dark-colored paints are also more likely to blister due to high temperatures because they absorb more heat.
High Humidity Levels
If you have ever wondered why the walls around your bathroom or kitchen look bumpy, then high humidity levels are your answer. This happens in my kids’ bathroom, which is actually why I started researching this article!
Water vapor can seep through the paint and weaken its adhesion to the wall. This causes it to lift off the wall and form bumps.
Older walls are more susceptible to this because the paint is aged and may have begun forming hairline cracks, allowing easy moisture penetration.
Applying Latex Paint Over Oil-Based Paint
Latex paint doesn’t adhere to oil-based paint, which may cause bumpy walls.
Before applying latex paint to a wall that has been covered in oil-based paint, the surface needs to be adequately prepared through the following steps:
- Cleaning the wall
- Scraping off the top coat
- Applying a suitable primer (made for latex paints)
- Applying a fresh coat of paint
Also, on a related note, check out my article on why your wallpaper appears bumpy. You’d be surprised how seemingly same problems can have different underlying causes.
How To Fix Bumpy Walls
Fortunately, bumpy walls can be resolved. The following guide will help you fix those blistered walls and hopefully prevent it from happening again.
1. Clean and Dry Your Walls Thoroughly Before Painting
Cleaning your walls before painting removes any particles and grime that may cause bumpy walls. You can clean the old-fashioned way using a sponge and water or a power hose. Grease-cutting agents may also be necessary when cleaning.
After cleaning, wait until the wall is completely dry before painting because painting over damp walls will leave you with bumpy walls.
2. Avoid Painting in Hot or Humid Weather
Whenever possible, wait until the weather is favorable to paint your walls.
For water-based or latex paints, ideal temperatures range between 50 and 80 degrees Fahrenheit (10 to 26.7 degrees Celsius). For oil-based paints, ideal temperatures range between 40 to 90 degrees Fahrenheit (4.4 to 32.2 degrees Celsius).
A relative humidity of between 40 and 50 percent is perfect for painting your walls. When there is a substantial variation from this range, it may affect how the paint dries.
3. Improve Air Circulation To Dry the Paint Adequately
Using a fan to dry the paint has two advantages — improving airflow and reducing relative humidity levels. This way, the paint dries off faster. However, for this to work, you need to position the fan properly and adjust to a moderate airflow setting.
Another way to improve airflow and prevent bumps on a freshly painted wall is by opening the doors and windows.
4. Apply Primer and Let It Dry
Primers protect your walls from blistering and peeling paint because they give the wall surface some texture, making the paint hold better. When you apply a primer before painting your walls, you will need fewer coats (remember that overpainting also causes bumpy walls).
Some paint brands have primers already mixed in, so you don’t need to add more.
Regardless of the type of walls your home has, priming before painting is recommended as a good practice.
Let the primer dry properly before painting over it. Three hours should be a sufficient waiting time.
5. Scrape Off Bubbled Paint and Apply a Fresh Coat
With the above preventive measures in place, you can now scrape off those ugly blisters using sandpaper and apply a fresh coat of paint.
This Youtube video covers a practical tutorial on the proper procedure for removing bubbled paint:
If only a small part of the wall was affected, you don’t need to scrape off the entire wall. But, if a large part was damaged, you may need to redo the entire wall. The good news is that if you follow the recommended steps, you won’t have to redo your walls soon.
Fixing blistering walls should start by identifying the root cause. Often, hot and humid weather is to blame. Another major contributing factor is following poor paint application procedures.
When the root causes are resolved, scraping off the damaged spots and applying a fresh coat of paint is likely to prevent the recurrence of bumpy walls.
Lastly, invest in a good quality paint brand to prevent the same thing from happening again!