How to prep exterior walls for painting

As a decorator of over 20 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 very quickly listing the tools and materials you’re going to need, followed by briefly touching on each step. I’ll then go into more details about each step.

Follow this advice and your wall will be properly prepped, and ready for you to paint.

How to prep exterior walls for painting

Tools and Supplies

Scrapers – Click here to see online prices.

Paint brush – Click here to see online prices.

Paint Roller – Click here to see online prices.

Masonry filler – Click here to see online prices.

Stabilising solution – Click here to see online prices.

Fungicidal Wash – Click here to see online prices.

Steps

Remove Loose Paint

No matter how good your new paint system is, if the previous coats are failing, then the finish paint will also fail. You should remove as much loose material as you possibly can.

Repair Defects

If there are any hairline cracks or holes, then this is the time to repair them. Use a good-quality cement-based filler and adhere to the ‘curing times’ before painting.

Stabilise The Wall

Any loose or powdery material needs to be stabilised. If you use Zinsser Peel Stop as your stabiliser, then the edges of any flaky paint will be glued down, and your paint finish will last longer.

Clean

Paint will not stick to dirt, and mould needs to be treated. Use a fungicidal wash to get your wall clean. Once you’ve done this, you’re ready to paint.

Getting Rid of the Old Paint

Not all existing paint necessarily has to be removed, but any loose material does. After all, no matter how good your new masonry paint is, if the paint underneath fails, so does the paint on top. So, the first thing you need to do is remove as much loose paint from your wall as you possibly can. I’d recommend buying a good quality scraper and really getting stuck in! Do not be tempted to use a wire brush, as splinters can remain in your wall and rust, causing damage to your painted wall further down the line.

Repairing your Exterior Wall

If you have imperfections, don’t worry, they can be repaired!! Larger holes can be filled using render if needed, however Toupret Murex 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.

Priming and Stabilising your Wall

Bare masonry DOES NOT need to be primed 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.

Cleaning your rendered 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 your exterior wall. Available online by clicking here.

Click here for a guide to the best masonry paint.