plywood discolouration - mold or just stain?

[dobster]

Hello kind people..

I made some drawers and a work bench in the storeroom a few years ago. The top is tas oak and the drawers are hoop pine plywood.
I sealed everything with Organoil exterior decking oil because I had a tin left over lying around - the space is a store room under the stairs after all. I, perhaps naively, thought anything designed for outdoor use would be fine for an indoor job like this.

491959

The plywood is now showing some discolouration all over the place, small reddish blotches, anywhere that didn't have something sitting on top of it.

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I first assumed mould and set about cleaning it, then sanding back with 80 grit with the intention of removing it all and applying a new sealer.
Lots of sanding later and I can barely get into it.

It doesn't look like the mould or fungus I'm used to seeing on wood. Any chance its some chemical reaction instead?
It appears on the top plywood shelves as well. Hasn't appeared to get into the surrounding red cedar trims of the house...

Any suggestions how to manage this would be much appreciated. Hoping I don't need to rip everything out and rebuild - the drawers are working fine still.

Many thanks
Stu

[BobL]

You could try some Oxalic acid - valuable from Bunnings as Rust and stain remover

https://www.bunnings.com.au/diggers-2kg-rust-and-stain-cleaner_p0960276
They seem t0 only sell it in large packs and at $30+ its not that cheap for something that you might just want to try.
Pity you are not close or I would just give you small amount.

May sure you try it out on a "not very visible spot" in case it makes it worse. :)

[wood spirit]

Doesn't look like any kinda mould -More water stains. would expect them to sand out -depends how deep they have gone. Natural finishes tend to suck unless under perfect circumstances.

[jack620]

I second the oxalic acid suggestion. Or some other form of chemical bleach.

I caution against using 80 grit on soft plywood. You'll very quickly sand through the top layer of ply and then your staining issue will be the least of your problems.

[Arron]

Deck Cleaner is basically oxalic acid and will work just as well, but is much cheaper. Test in a small area first.