Spotty discolouration to render ... west country problem!?

[GrinAndBearIt]

Our nice, crisp newly rendered walls (straight off fresh blockwork) suddenly look (five days after completed) like they've been subject to a massacre. Loads of weeping brown/bronze spots everywhere. Render mix was cement, lime & plasterer's sand (1:1:6).

Now only noticed this today, and preliminary research seems to suggest this does crop up from time to time here in the west country. Someone even mentioned it was something to do with sand sourced from near heather!? So does anyone have any top tips on how to stop this in its tracks ahead of us applying the top acrylic coat? If some sort of remedial work isn't done now, I'm convinced the final finish will be compromised, probably quite badly given we are going for a very light finish ...

Wide view

Untitled

• GrinAndBearIt
• 23 May 2014
• 2

Alarming development in new render

Close up

Untitled

• GrinAndBearIt
• 23 May 2014
• 2

Close up of spot ...

Thanks for any tips or advice ... very gratefully received!

Apologies for copying this over from the other thread re: joining new render to old, but thought it worthy of a different heading!

 

[roughcaster]

That is "rust". It comes from iron pyrites in the render/sand/chips etc. Hard to say if it will bleed through the finish. You'll definitely need to ask the contractor. Tricky one!!

 

[GrinAndBearIt]

Thank you! Searched for pyrites in render, and have found other examples of this happening. Actually ... bizarrely ... looking at it again I see it was mentioned in a thread in which you ('roughcaster') participated 4 years ago!

Apparently the best approach is to dab on a generic rust eater product, then a solvent based undercoat, and then something like Zinsser 123 to prevent any further rust formation ....

One thing I have noticed is it only seems to affect the walls that are well in the way of the weather. So I wonder whether I should try and spray/drench the other walls just to try to promote any rust stains so they can be dealt with now ... rather than waiting until they may show up years from now when the acrylic final layer is on!

 

[GrinAndBearIt]

Actually ... no ... not rust.

Had the head technician from the aggregates company come to site - turns out it's LIGNITE contamination in the sand - basically a very soft low-grade type of coal. Now apparently this won't come through a 3mm top coat of an acrylic render ... but if anyone knows differently, then it would be great to hear from you!

Thanks y'all.

 

[roughcaster]

Glad you got it sorted out GrinAndBearIt. All the best.

 

[Gilly100]

Grin and bear it. We are having same problem I could cry!! How did your rectification of walls turn out. Would really appreciate if could chat to u. Thanks

 

[roy c]

Find out where the sand came from and take it up with the suppliers of rendering sand that gave you this problem!!!!! Also I would blame the plasterers for not picking up on it !!! A good plasterer should of picked up on it!!

 

[Gilly100]

No I don't blame the plasterer at all. You would think it is a black stone until u crush it. Wouldn't see it until it is in a rubbed up wall which starts drying out. Sand supplier has admitted fault but we now have hassle of getting windows protected to start chipping it all off!! It's so depressing. Plasterer did fab job.

 

[23vc]

I had one single spot of this appear in a wall I rendered recently, the paint totally hid it, nothings bled through.

 

[Gilly100]

We have hundreds of spots though. Saw a wall done 2 years ago the top was rubbed up and was speckled with lignite. Top of wall today looks dreadful all blowen looks like bad acne pits!! Uf it was only a few i would def just seal and paint.