Stucco Patch Has Rust Spots Showing, I Need To Paint

Danny had a stucco patch done around a window about a year ago and recently noticed staining in one of the corners. He’s concerned that if he paints over it, the stain might bleed through over time. He’s also wondering whether the unpainted stucco could be absorbing moisture and causing rust from the metal lath to seep through.

Question from Danny: About a year ago, I got some stucco work done surrounding a window. Recently, I noticed some staining in the corner. There is also a horizontal hairline crack running through the stain.

I did not get the new stucco painted yet because I was/am planning to repaint the whole house. My concern now is that if I paint the house, then the stain would eventually bleed through the new paint.

I don't see any leaks on the inside of the window and no obvious water intrusion on the outside besides the stain. Is it possible the stain is due to the new stucco being unpainted, absorbing water, rusting the lath, and seeping back out?

Any other ideas on what could be causing this and how I can prevent it from staining when I finally paint the house? I appreciate any insight you can provide. Thank you.

Stucco Repair Around Window
Stucco Repair Around Window 3
Stucco Repair Around Window 2

Answer: Hello Danny! Your concerns are valid, and from what I can see, the stucco patch should have been painted to seal it, shortly after it was done or at least primed.

While painting isn’t a requirement for traditional cement-based finishes, the walls around it are painted and the patch should have been too, in my opinion. This would have likely prevented this from happening in the first place, but no matter, it can still be addressed.

Looking at the surrounding area, there are clear signs of high moisture levels. A few things stand out:

  • The rusty corner aid on the left side suggests prolonged exposure to moisture.

  • Water staining running down the porch on the right side shows that water is getting trapped in that area.

  • Moss/algae growth on the concrete further confirms ongoing moisture issues.

How to Prepare the Area Before Painting

Before you paint, I would recommend pressure washing the entire area, including the window, to remove any dirt, rust, or organic growth. If the rust stains remain after cleaning, apply a rust-preventative primer before painting. This will help prevent any bleed-through when the final coat of paint is applied. Just caulk the crack before painting and blend it in with a float or sponge.

Is the Stucco Installation a Problem?

As far as the stucco installation itself, nothing in your photos suggests any major issues that would cause long-term problems. The stain is likely due to moisture interacting with the unpainted stucco and possibly some oxidation from the metal lath. Painting will help seal the surface and protect it from further water absorption.

Let me know if you need any clarification!

About the author

The Stucco Guy

My name is Ryan and I have been a licensed stucco contractor for many years and I feel that there is a huge "knowledge gap" when it comes to stucco, in particular. I hope you find the information here useful, and if you have a question for me fill out this Q & A form, so I can answer those questions better. Thanks for stopping by!

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