Cracked Plaster Walls? Here’s How to Repair Them Properly (And Stop Them Coming Back)

A crack in plaster is easy to ignore…

Until it keeps reopening.

Then you fill it.

Paint it.

And a few months later…

👉 It’s back.

That’s because a lot of plaster cracks don’t fail because of the filler.

They fail because they weren’t repaired properly in the first place.

This is how to identify what you’re looking at — and repair plaster cracks the way they’re dealt with on site.

First — Not All Plaster Cracks Are The Same

Some cracks are cosmetic.

Some suggest movement.

Knowing the difference matters.

Hairline Cracks

These fine cracks are common and often show up through natural movement, drying shrinkage or minor seasonal expansion.

Very common in older plaster.

Often repairable.

Cracks That Keep Reappearing

This is where it gets more interesting.

If you’ve filled a crack before and it keeps returning…

👉 That’s movement.

And standard filler alone often won’t hold it.

It needs reinforcing.

That’s where the repair changes.

Wider or Deeper Cracks

Once you get towards cracks around 1cm deep or deeper…

This is no longer a skim-over repair.

It needs rebuilding.

Sometimes that means filling in stages.

Sometimes it means checking what’s happening behind the plaster.

Especially if the crack is growing.

The Biggest Mistake People Make

They fill the visible crack only.

They don’t repair what’s underneath.

That’s why it comes back.

👉 You need to rebuild the crack, not just hide it.

How To Repair a Cracked Plaster Wall Properly

1. Rack The Crack Out

First:

Don’t fill over the surface.

Open the crack up.

Rack it out as deep as it will go.

Then feather the edges slightly with a scraper so the repair can blend back properly.

This gives the repair something to grip.

Without this—

Filler is just sitting on the surface.

2. Remove Every Bit of Dust

This matters more than people think.

Use a brush to clean out the crack.

Better still—

Use a blower to get right into it and remove all loose dust.

👉 Dust is bond failure waiting to happen.

Clean crack = stronger repair.

3. Seal The Crack First

Once clean—

Soak the crack with Zinsser Gardz or equivalent.

This binds dusty edges and stops the plaster sucking moisture from the filler too quickly.

Let it dry.

This step gets skipped all the time.

It shouldn’t.

4. Use The Right Filler For Movement Cracks

If the crack keeps reopening…

Use Toupret Flexfill.

Very good product for this.

If the crack is around 1cm deep, Flexfill works well.

If deeper than that—

Build depth first with Bonding plaster (or drywall repair as needed), let that dry…

Then apply Flexfill.

Important:

Don’t spread Flexfill beyond the crack itself.

👉 Keep it in the gap only.

It contains fibres and creates a very strong rubbery bond…

But it’s horrible to sand as a finish coat.

Use it where it’s needed.

Nowhere else.

5. Apply a Finish Filler Over The Repair

Once the movement crack is dealt with…

Apply a smooth layer of interior filler over the area.

I’ve had good results with Toupret Skim Filler.

This gives you your finish coat.

Not the Flexfill.

6. Reinforce It With Fibre Tape

Now bed fibre scrim tape into the wet filler.

Push it in until properly embedded.

Not sitting proud.

Embedded.

That reinforcement is what helps stop the crack telegraphing back through.

Let that dry around half an hour.

Then go over it again with another skim of filler…

Slightly wider than the first pass.

Feather it out.

Now the repair disappears.

7. Let It Dry Properly — Then Sand

Let everything dry fully.

Then sand smooth.

Take your time here.

A rushed sand always shows after paint.

Why This Repair Holds Better

Because you’ve done four things most people skip:

✔ Opened the crack properly✔ Stabilised it✔ Reinforced it✔ Rebuilt it in layers

That’s why it lasts.

When You May Need A Professional

Sometimes a crack isn’t really a plaster problem.

It’s movement.

If the crack:

• Keeps widening

• Reopens quickly after repair

• Runs diagonally across walls

• Appears with bulging or sagging

• Shows signs of moisture or structural movement

👉 Get it assessed.

Especially if it looks serious.

If it looks serious…

It probably is.

🔚 Final Thought

Most plaster cracks don’t come back because filler failed.

They come back because the crack was only covered…

Not repaired.

Open it up.

Stabilise it.

Reinforce it.

Build it properly.

And you fix it once.

❓FAQ: Repairing Cracked Plaster Walls

Why does my plaster crack keep coming back?

Usually because it was filled but not reinforced, or the movement causing it wasn’t dealt with.

Should I fill over a plaster crack?

No — rack it out first. Filling over the surface rarely lasts.

What filler is best for moving cracks?

For recurring cracks, Toupret Flexfill is very good.

What if the crack is too deep for filler?

Build depth first with bonding plaster, let it dry, then repair over that.

Do I need scrim tape over plaster cracks?

For cracks likely to return — yes, fibre tape adds strength and helps stop re-cracking.

Why use Zinsser Gardz before filling?

It binds dusty edges and improves adhesion inside the crack.

Can I use Flexfill as a finish coat?

Better not. It’s strong, but unpleasant to sand. Use a finish filler over it.