How to Fix Cracks in Walls – A Full Guide

If you’re going to redecorate a room, then you should make repairs to the walls before you start, including cracks. In this blog, I’m going to explain how to fix cracks in walls.

It’s a relatively straightforward process and you should find it quick and easy if you follow the advice I give you. There are a couple of different methods; general hairline cracks are a lot easier to deal with, but if the crack is reoccurring due to movement, then you need to carry out a slightly different process.

Feel free to skim the blog and just pick out the information you’re looking for. I’ll link to a couple of websites so you can buy the materials needed, but you can buy all this stuff from thousands of stockists, so feel free to shop around.

How Can You Tell if a Crack is Structural?

I’m going to focus on how to fix surface cracks in walls, but I know big cracks can be scary. Cracks caused by structural issues like subsidence are a whole other ball game, and you really need to consult a builder to diagnose the root cause.

If the crack is structural, then it won’t just be plaster deep; the crack is likely to go all the way through the plaster and the block/brick that the inside of your wall is made from. Sometimes you can even see the crack on the outside of the building too. Or different cracks outside, but in a similar place to the crack you have inside. Anything to indicate that the whole house is moving, and not just the plaster inside.

99 times out of 100, you don’t need to worry about subsidence. If you do have a particularly big crack and you’re unsure how deep it goes, you can always open it more with a filling knife or scraper to see. This may create a bigger hole to fill, but it’ll be worth it for piece-of-mind.

How to Fix Hairline Cracks in Walls

As mentioned, there are different methods depending on whether you have a hairline crack to fix or a reoccurring crack. If you’re in any doubt, all you need to do is put pressure on each side of the crack in turn. You will be able to see the movement if your crack is reoccurring. If so, skip ahead to the relevant section.

You will need

• Sandpaper
• filling knife
• A good quality filler

Open the Crack

I know this sounds counterintuitive, but you need your filler to penetrate the crack to fix it. So, insert the corner of your filling knife or a scraper into the crack and carve it open. It only needs to be a couple of millimetres deep, so be careful not to be too aggressive with it.vily first.

Abrade the Crack

The wall is usually slightly raised at each side of the crack, particularly after you’ve carved it with a filling knife. Abrade it slightly until everything is flat. This makes it possible to fill it flush and avoids you getting bits in your filler.

Fill

Mix a good-quality filler and apply it to the crack. Run the filler along the crack, rather than passing the filling knife across it. Big cracks can take more than one fill, so allow the filler you’ve applied to dry, and apply an extra coat of filler if needed.

Abrade the Repair

You need to sand across the repair rather than rubbing your sandpaper up and down the crack. If you sand the wrong way, you’ll pull too much filler out of the crack and you’ll need to fill again.

Paint

Painting over filler does take a little bit of consideration. You should dust the area first, and then prime with a diluted coat of matt emulsion or some Zinsser Gardz.

Painting Over Hairline Cracks

How to Fix a Reoccurring Crack

If there is movement in the plaster (usually between two plasterboards, or around a previously patched area), then you need to take the following approach.

You will need:

• Scrim tape
• Sandpaper
• filling knife
• A good-quality filler

Open the crack

Use the corner of your filling knife to open the crack and then abrade so the surface at either side of the crack is flush with the rest of the wall.

Apply Scrim Tape

This is what’s going to stop the movement, which stops the crack from reoccurring. Run some scrim tape (available online here) over the length of the crack. It’s self-adhesive mesh, so it knits both sides of the crack together.

Apply a Coat of Filler

This method always requires two coats of filler, but they’re applied slightly differently to each other. You should work the first coat of filler through the scrim tape into the crack, but you need to be careful not to damage the scrim tape. Just take your time, be gentle, and don’t be too stingy with the amount of filler you use. Allow to dry.

Apply a Second Coat of Filler

You don’t need to be as gentle with the second coat of filler because the scrim tape will already be partially covered. This time you need to cover all the scrim tape with the filler, and ‘feather it off’, which just means a gradual slope of filler from the scrim tape down to the rest of the wall. Filling like this blends the slightly raised scrim tape with the rest of the wall.

Abrade

Use a fine sandpaper and carefully abrade the filler until the surface is smooth.

Paint

Dust the filler and then prime with a coat of diluted matt emulsion or Zinsser Gardz. Your crack has been repaired and you’re free to carry on with the rest of the decorating.

A Video Demo

FAQs

Which cracks in walls are serious?

Structural cracks caused by subsidence are very serious. There is usually evidence both inside and outside of the property if your cracks are structural.

What is best to use to fill cracks in walls?

Any good-quality interior filler can be used to fill cracks in walls, however you should avoid ‘multipurpose fillers’ because they contain cement which may flash through the paint.

Can I just paint over cracks in walls?

You can paint over hairline cracks if you use a specialist paint like Bedec Interior Flex.  This bridges the cracks and withstands movement, which prevents the cracks from coming back.

Can you plaster over cracked walls?

You’d normally apply scrim tape over the crack and then plaster over that. This prevents the crack from coming through your new plaster.

Should I PVA cracks before filling?

PVA is a great stabiliser for powdery surfaces. It’s not usually needed when fixing cracks on walls, but if you do use it, ensure you dilute it heaily first.

Final Thoughts

I know it’s a bit of a cliché but prep really is the most important part of every decorating job. Now you know how to fill cracks on walls, you’re halfway there. Don’t forget though, if you think you have structural damage then you should get it checked out as soon as you can. Problems like that tend to get worse if left unchecked.