How to prep exterior walls for painting

As a decorator of over 22 years, painting masonry walls is something I do on a regular basis. But ask any decorator, and they will tell you that the prep is the most important part of the process. With that in mind, I thought I’d take the time to sit down and put together a simple guide on how to prep exterior walls before painting. I hope you find it useful.

I’ll start by explaining why proper prep is so important, then I’ll quickly list the tools and materials you’re going to need, followed by taking you through the steps. Feel free to skip ahead in the blog and pick out the information you need.

Follow this advice and you can’t go wrong.

Why Preparing an Exterior Wall is so Important

The paint on the outside of your house needs to do a lot more than the paint inside. Yes, it needs to look good, but it also needs to repel water, withstand UV rays, cope with changing temperatures, and withstand algae growth.

Masonry paints come with a guide on how long they should last. Emperor Paint can last 25 years without showing signs of deterioration (click here for details). However, because you’re asking the paint to do so much, you need to make sure it has a good base.

If the existing paint on your wall is flaking and continues to fail, then your new coating will fail too. If the wall is dirty when you paint it, you will have adhesion issues, and your lovely new paint will flake and peel over time. Any algae growth that isn’t properly treated will bleed through the new paint and stain, or worse, carry on growing.

You are asking your exterior wall paint to do a heck of a lot, so the least you can do is give it the best possible chance.

How to prep exterior walls for painting

Tools and Supplies:

• Scrapers
• Paint brush
• Paint Roller
• Masonry filler
• Stabilising solution
• Fungicidal Wash

Sheet up

Cover windows and doors so they don’t get damaged. I use Indasa Cover roll and Masq Painters Tape for this, but there are other good products on the market. Then cover the floor around the area you’re working.

Covering the floor will help protect it from damage in case you drop any tools or masonry, and it helps when you come to clean up afterwards. You’ll be able to pick a lot of the debris up in the sheets and put it directly in the bin, rather than trying to brush a large area.

If you are using ladders to access high parts of the wall, then you need to be conscious that the sheets can cause a hazard. Dust sheets can reduce friction at the bottom of the ladder and cause it to slip, so always peel the sheets back and foot the ladder directly on the ground.

Get Rid of the Old Paint

If your exterior wall is previously painted, then you should remove any paint that is flaking or bubbling. You don’t necessarily need to remove all the existing paint, but if you leave some that fails further down the line, your new paint will also fail.

Use a good-quality scraper and go over any bits you think aren’t stuck solid. You can use a wire brush too, but if you do that, you need to be careful that splinters don’t get embedded in the wall, otherwise you will have issues with rust bleeding through your paint after a few weeks.

Repairing your Exterior Wall

If you have imperfections or cracks on your wall, don’t worry, they can be repaired!! Large holes can be filled using render if needed, however Toupret Rock Solid masonry filler can manage most repairs. It dries rock solid, so you know your wall repair will last, available online by clicking here. Hairline cracks should be carved out slightly so your filler can penetrate them, rather than sitting on top, which can cause your crack to reappear further down the line.

If your exterior wall is textured, you can finger filler into your cracks, then wipe the excess away with a paintbrush dipped in water. This will blend your repair with the rest of your wall. Or, for larger repairs, you can add texture to your filler by dabbing it with a wet carrier bag filled with newspaper.

Making repairs isn’t just about aesthetics; it helps seal the wall and prevent moisture from penetrating, so your paintwork lasts longer.

Clean the wall

You don’t need to go over the top with cleaning. But, if your wall is very dirty and you don’t clean it, you may be left with an adhesion issue between your new paint and rendered wall. All I do is hose it off and give it a scrub with a brush. Better still, use a power washer. Remember to allow your wall to fully dry before painting.

If you have green algae or moss on your wall, then you need to use something like Dulux Trade Weathershield Fungicidal Wash to kill it. This prevents the algae from growing back after you’ve painted. Just apply a diluted coat of the fungicidal wash to the wall and allow it to dry overnight. Available online by clicking here.

Click here for a guide to the best masonry paint.

Prime and Stabilise your Wall

Bare masonry DOES NOT need to be primed in most cases, unless it is chalky and unstable. However, if your surface is chalky, then a coat of Zinsser Peel Stop is the way forward. This is a water-based product which remains breathable when dry. It binds loose paint and stabilises the surface underneath, ready for you to apply your paint. Available online here.

The only other thing that may need priming are rust spots. Ideally, you don’t want any metal in your wall, but maybe you may have a nail or two so you can hang stuff on, or something within the wall you can’t get to. Just spot-prime any bits with oxi primer to stop the metal from rusting and bleeding through your finished paint film further down the line.

A Video Demo

FAQs

Should you pressure wash exterior walls before painting?

Pressure washing is a good way to clean an exterior wall and remove flaking paint, however you can saturate the wall if you’re not careful. This isn’t a major issue providing you allow plenty of time for it to dry before painting.

How long should I wait after pressure washing an exterior wall before painting?

This depends on the weather, but 2 days is the standard. The wall needs to be fully dry before painting to avoid issues.

Can I wash walls with washing-up liquid before painting?

You can use washing up liquid on exterior walls, but ensure you rinse the walls afterwards to avoid leaving a residue.

Can you paint over dirty exterior walls?

In short, no. Any material on the surface will prevent the paint from adhering to the wall and can cause it to flake over time.

Final Thoughts

Prepping an exterior wall isn’t rocket science, but it can be time consuming and hard work. Get it right and use good-quality paints, and unless there’s an unforeseen problem, your paintwork will last for years. Get it wrong or use cheap paints, and you’ll need to repaint again after a year or two.

For me, exterior painting is all about longevity. There’s no point in cutting corners if it means more work in the long run. For that reason, you should follow the advice laid out in this blog.