Painting external wall, but wall has turned red

[Supernatural323]

Good morning,
This is my second post regarding painting my house. I had very useful tips on my first set of questions, but thought I should start a new thread with my next question.

I am preparing to paint the south facing external wall, the wall is roughcasted about 5 years ago, and never been painted. The wall is in very good condition apart form the fact it has turned red. I see loads of houses around with the same problem, always the south wall.

I have pressure washed a section to see how easy it is to remove, and thankfully it comes of very easily.

My questions are:-

Why did the wall turn red? is the colour coming out of the wall or is it turning red on the surface with the rain or something?

Can anyone advise what I should be doing, as in, is it OK to just clean the wall and then paint over with paint (first coat 20% water)

Many thanks in advance

JJ

 

[dcdec]

Could be a form of fungal growth, its commonly red in certain areas, if it is a fungicidal wash should do the trick. Hosing it off won't kill it, you can also use a bleach solution but its very difficult to prevent run off into the surrounding environment.

 

[Supernatural323]

Please see picture of wall. Can you tell from pic if it is fungus as it is everywhere? If not what is it? Just below the roof where the rain does not get to, the colour is normal.

Untitled

• Supernatural323
• 15 Sep 2014
• 1

 

[dcdec]

Unless anyone has a better idea i'd still say its fungal

 

[misterhelpful]

Now that I can see a picture, I am almost certain it is an algae of some kind, as I mentioned in your previous thread.

The way it appears in areas where rainwater is concentrated, (i.e., in streaks where it runs off the edges of the roof tiles) and doesn't appear in the dry area directly under the verge boards is a common sign. The thing that has thrown me is that you say the wall is south facing - most algae forms in damp areas out of direct sunlight, but I suppose different types thrive elsewhere.

Are you near the coast in Pembrokeshire, as I suppose a form of sea algae could be a contributory factor?

Other than that, the only thing I can think of is dye running out of red roof tiles, if that's what you have.

The best advice I can offer is to assume it is an algae and treat it as has been described previously. Remember to rinse off any fungicidal wash/bleach solution before painting.

 

[Supernatural323]

Thanks misterhelpful.

I'm about 40 miles from the coast and I have slate roof. I was hoping it wasn't algie, if not cleared properly, it would probably show through paint?

Are you a profesional in this field? What fungus killer should I use? Is bleach best option? And how much should i dilute it by?

What is best way of using the stuff, if spraying? Can you recommend something? Cheaper the better.

Many thanks mate

 

[misterhelpful]

I am no expert in the algae field but, as a decorator of 20+ years, I killed/cleaned off more than I care to remember.
If what you have is a form of algae, and you paint directly over it, you could find that it continues growing and will break through the paint film causing it to flake and peel. You could also find that, even if you kill it first, it will still form on top of the new paint, although that isn't as bad because you can just kill it off again.

On large areas like you have I used a fungicidal wash that could be diluted and would put it into the bottle sprayer of a pressure washer, spray the entire area, leave for the recommended time and then pressure wash it all off. On smaller areas I would use one that was applied neat and then washed off a while later (generally used Everbuild but most brands work perfectly well). This always seemed to work but I must admit I have never encountered red algae in my neck of the woods.

I wouldn't use bleach on such a large area as it can be a bit harsh on the surrounding environment, as has already been mentioned. That said, fungicidal washes aren't exactly environmentally friendly.

 

[ic1927]

The best stuff I've used for getting rid of algae is Bond-it Moss Stop. Dilute it as per instructions, put it in the roller scuttle and using a deep pile roller with a 5 metre extension pole and roll the whole area and brush in what you can't get at with the roller. Leave it for a few days, make sure the weather is dry, and then jet wash it off and it's ready for painting. It's organic and won't harm plants and pets. Brilliant stuff.