How do you seal an exterior wall before painting

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When it comes to painting masonry walls, you should always check first to see whether or not you need to seal them. Sealant is used on most masonry that either has not been painted before or that has been scraped back to exposed brickwork before painting along with primer and undercoat.

What does sealing exterior walls do?

Sealing an exterior wall prevents the topcoat (the paint) from soaking into the masonry walls. It also, to some extent, can help to provide a water-resistant layer between the masonry and the top coat of paint in order to keep moisture out and away from the walls. Along with primer, this can help to prevent mildew and mould and can reduce loose or flaking paint. Sealant can be used on its own in place of primer but is often used alongside primer, which holds its own benefits for masonry walls. Sealing is often done by using a thinned coat of paint and is done on porous and chalky surfaces like bare wood, and cement render (sand and cement) that is often used for exterior rendering of homes.

Some surfaces, like new surfaces, bare walls and sound surfaces can be damaged if you use sealant before painting them. It is worth checking and doing ample research when planning to paint your walls. Glossy sealers, when used unnecessarily, can cause flaking paint as much masonry paint is designed to be breathable, and so moisture passes through and collects on the sealer.

How to seal exterior walls

Sealing exterior walls is a relatively simple process, and when done well can make painting your exterior walls significantly easier! The sealer prevents the new paint from soaking into the masonry walls, meaning that you have to use much less paint to saturate the wall.

To seal your wall, you must:

  1. If the wall has been previously painted, remove existing paint. This is likely to be loose or flaking paint, and you can remove it easily with a wire brush. Remove all of the old paint using the brush, ensuring that the wall is completely bare.
  2. Using mildew cleaner, clean the substrate from the top down. You can use a sprayer to do this and should ensure that you move plants and stones away from the bottom of the walls to keep them protected from the sealant.
  3. Cover anything around the wall that you don’t want to be sealed, this includes windows, window frames, doors, and the floor. If it is not covered, it runs the risk of getting paint on it.
  4. Ensure that you have the right protective gear to protect your eyes and hands from the chemicals within the paint. You should have protective goggles, gloves and even a mask.
  5. Load up a pump sprayer with a sealer and spray from the top down again. If the paint begins to pool, use a roller to spread out the drips and pools across the wall. Leave the sealer to dry, and then do the 2nd coat as per the manufacturer’s instructions. Check to see how many coats your sealant requires!

What else do you have to do to exterior walls before painting?

In preparation for painting your walls, there are a few things you need to do in addition to sealing the masonry. Just like with painting interior walls, you need to clean the walls before painting. With interior walls, you can do so with warm water and a sponge, and with exterior walls, you can use clean water and a cloth or sponge, or even a pressure washer to clean the wall and remove any dirt.

Once your outside walls are clean, you need to use sealer on them – this is especially necessary if your walls are chalky, porous or they are not smooth. After using sealer on them, they should not remain powdery as the walls should now be sealed. You now need to prime the walls; primer works to block out stains, stops rust from forming, smooths out imperfections, and many primers also work to reduce the risk of mould and mildew from forming. The best primers available on the market at the moment are:

You can simply follow the manufacturer’s instructions when priming your walls to determine how many coats to use on your concrete floors or masonry walls. It is worth purchasing and using a primer with protection against moisture and mildew, especially in cooler regions and in coastal areas affected the most by humidity and moisture. These problem areas are particularly prone to mildew and moisture issues that can cause your painted surface to flake, ruining your quality finish, regardless of how many coats you used.

As soon as your walls are primed and the primer is dry, you can then use an undercoat. Undercoat makes sure that your walls are fully primed and ready for your masonry paint by filling any minuscule imperfections. It also works as a lightener, allowing you to paint your walls lighter colours.

Once you have finished your preparation, you can then begin to consider which paints you should use. You need to first of all pick colour and finish, and then do your research into which paint works on the materials you need to paint – for pebbledash walls the best paint will be different than on wood, concrete or metal.