Cleaning redwood fence - percarbonate oddity

[9kckzqii3 14]

Hi there,

I have an old redwood fence that hasn't been cleaned up for a good 5 years. The fence looks dull grayish.

Last weekend I decided to do a good cleanup. I used percarbonate solution, prewet the surface briefly, applied it with a garden sprayer, and rinsed it off with a power washer (@ medium pressure).

As it dried up, the fence wood appeared brownish in color. I thought it needed a 2nd application. I reapplied. As it dried up again, the wood color was lightened up a bit, so I was pleased.

The day afterward, the wood started to look pale greenish.

Would anyone know what this is? Tannins from the redwood?

I wonder what I can do now to clear up the pale greenish color on the redwood?

Thanks.

Edited May 25, 2011 by 9kckzqii3

[Beth n Rod 1,279]

You need a mild acid to neutralize with. (oxalic will work fine)

Pre-wet the fence lightly, apply the acid, let dwell, then rinse thoroughly. The wood will return tot the natural pH.

Beth

[Doug Black 18]

You need a mild acid to neutralize with. (oxalic will work fine)

Pre-wet the fence lightly, apply the acid, let dwell, then rinse thoroughly. The wood will return tot the natural pH.

Beth

True, plus it reverses the darkening percarb can do.

[9kckzqii3 14]

Thx for the info.

Also I noticed as I ran the power washer to rinse off the percarbonate, tannic acid (dark brown) was leeching out and dripping. At other times, the dripping was in pale green after I scrubbed the wood surface.

Is this normal?

Edited May 26, 2011 by 9kckzqii3

[9kckzqii3 14]

First time using percarbonate, is oxalic acid always needed after applying percarbonate? If the right amount of percarbonate was sprayed on the wood then no need to neutralize the percarbonate?

Thx.

Edited May 26, 2011 by 9kckzqii3

[9kckzqii3 14]

I'm trying to understand--if percarb is close to pH neutral, why would it darken the wood? What is it exactly doing to the wood that darkens the color?

Thx... .

[Celeste 341]

Redwood behaves differently. It's just the way it is.

[Greg R 82]

The pH of a percarb solution (not pH nuetral) brings tannin to the surface therefore darkening the appearance. The acidic base brightener will supress the tannins and bring back the natural color. There's some chemistry going on there..... We have always brightened after using a percarb.

[James 625]

Redwood brightens up better when almost damp. It also has the the most tannins.

[9kckzqii3 14]

Hi, thx for the info. Looks like percarbonate is not pH neutral.

BTW, wonder what concentration of oxalic acid (brightener) I should use?

I am seeing these products:

Zep Deck Cleaner Concentrate (6% oxalic) : to be diluted up to 20 times

Behr Brightener No. 63 1 gallon (5-10% oxalic) : to be diluted to 5 gallons

Thompson's Deck Brightener (6% oxalic acid) : no dilution required

Seems when diluted it'd only have 2% concentration of oxalic for Behr and 0.3% for 20 times dilution for Zep. Are these figures too low? Thompson's has 6% oxalic and not instructed to be diluted. So I'm a little confused here.

Thanks.

Edited June 11, 2011 by 9kckzqii3

[9kckzqii3 14]

Tried 0.3% Oxalic and noticed no improvement. Next tried 2% oxalic -- and it worked! Now wood looks brightened up and really nice!

BTW, I wonder if I can rinse *after* applying percarbonate and oxalic (saving one-step of rinsing):

- pre-wet wood

- apply percarbonate

- apply oxalic acid (to brighten/neutralize percarbonate)

- rinse

Also, I'm curious--would household vinegar (5% acetic acid) also work as neutralizer/brightener after applying percarbonate?

The reason I ask is that household vinegar is economical and readily available, and if the 5% concentration acetic acid is appropriate, there is no need to dilute -- just pour and spray.

There're also a couple of areas (like the top of the gate) where after all these still appear milky/light grayish. Do I need to reapply more oxalic acid, or reapply percarbonate?

Thanks.

Edited June 24, 2011 by 9kckzqii3

[James 625]

You can use vinegar. Milky grayish is where all the dead/ weather wood didn't come off. Buff it off manually.