Tannin? staining - Flag staining under oak tree

kipper · Post: # 31269Post **kipper** Mon Sep 01, 2008 11:22 am

Our 3 year old Indian sandstone patio is partly under an oak tree and many of the flags there have become black streaked/stained. I've tried various ways of removing the stains without success; power washing, bleach, propriatory stain removers....
Have you any ideas please?
thanks

Mon Sep 01, 2008 6:25 pm

did you seal the patio when it was new?
in general lithofin make the best products for natural stone care,best to contact their technical help department
LLL

RAPressureWashing · Mon Sep 01, 2008 9:51 pm

Kipper,
For tannin staining, you can use the mix that is on the stain removals page on the main web-site.
Out of interest what strenght bleach have you tried using?
Or we could come and clean this for you.

kipper · Post: # 31568Post **kipper** Fri Sep 12, 2008 9:49 am

Many thanks guys. I didn't seal the patio when it was laid and I used domestic strength bleach.
Cheers
Kipper

RAPressureWashing · Fri Sep 12, 2008 5:14 pm

kipper wrote:Many thanks guys. I didn't seal the patio when it was laid and I used domestic strength bleach.
Cheers
Kipper

The domestic bleach is not strong enough see Stain remove page on main website for what you need.

derekoak · Post: # 31674Post **derekoak** Thu Sep 18, 2008 12:03 pm

If it is tannin from the oak tree and the stain is black the tannic acid wiil have attached itself to iron atoms in the stone. If you can get oxalic acid it will replace the tannic acid. Ferrous/Ferric oxalate is not coloured so the blue/black colour should fade. I work with oak and we got some oxalic acid from an industrial chemist.

RAPressureWashing · Thu Sep 18, 2008 2:24 pm

Be very careful with Oxalic Acid, as it can permanently stain paving, esp concrete.
derekoak
I understand how you use Oxalic when working with wood, we do a lot of decking cleaning/restoration so we use either Oxalic or Citric acid as a neutralizer.

derekoak · Post: # 31678Post **derekoak** Thu Sep 18, 2008 4:12 pm

I have removed stain from our oak off bricks and concrete with oxalic acid and not noticed a stain, but I did not stay onsite for long. What happens to concrete?

RAPressureWashing · Thu Sep 18, 2008 4:22 pm

derekoak wrote:I have removed stain from our oak off bricks and concrete with oxalic acid and not noticed a stain, but I did not stay onsite for long. What happens to concrete?

It can "bleach" out concrete, iE leave spots etc
If you do have to use Ox, don't spot clean, you need to do the whole paver or surface.