Garden shed wood treatment questions...

23 May 2004
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United Kingdom
Over the last couple of years I've built two sheds/workshop. Having taken advice from here I coated the shiplap (which was vac treated) with up to 6 coats of tung oil..

On the first workshop all went well the first year, but having recoated it last summer (and treating the second shed likewise after build), both have suffered badly over the winter. Mould has covered most of the boards (the SW facing ones which take the brunt of the weather fared worse) and in the worst affected areas the wood expanded so much it started to 'round' (gaps were left between the shiplap on construction to allow for expansion), and some tiny 'cracks' have appeared. The wood is 'sound' though..

So if we ever get summer back, I need to get a protective coating on them..obviously I need to sand the wood quite thoroughly, but given that I don't want to use the oil again, some questions..

What do I need to do to the shiplap so that the oil that has been applied previously doesn't affect the new coating?...only one area is very slightly 'sticky'. Would a clean with white spirit suffice?

The reason for using oil in the first place was to try and keep the wood in it's natural colour, so is there a durable clear treatment that I could now use?

If a coloured treatment is better, what would be the best to use which only gives the wood a light shade of brown?

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Bloddy 'ell, where is this shed, Ben Nevis?

When you say mould do you just mean green algae type stuff you get on sheltered tree trunks or 'proper' mould?
If the wood is rotting I'd reclad it after finding out where you went wrong in the first place.

As you've oiled it, it's unlikely that any paint or stain will take until the oil weathers out, which could take up to *grumble, fzzznnn, shhhnnn* years.

Seeing as it pressure treated, it should last at least ten years unprotected anyway. If you wanted to preserve it without changing the colour you shoulda used clear wood preserver, something like cuprinol 5 star.

If the wood has rounded (cupped) then there's not much you can do now but unlikely to get worse. Proably caused by incorrect fixing positions, or too many nails restricting movement.
my Australian b-i-l commented when I showed him my linseed-oiled external hardwood, that they get mould on it in Oz.

I asumed it was the combination of warmth and humidity.

I treated my stuff with Cuprinol before oiling, so I am not expecting mould :confused:

Can you shows us some pics?
I've just uploaded some pics...


The wood is still 'sound', and the 'rounding' of the boards has disappeared now the wood has dried out a bit - and it only happened on the SW exposed side. Only a single row of fixings per board to allow expansion/contraction.
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is the black stuff the mould?

Is it growing on the surface of the oil, or the wood underneath it?
it's on the surface of the wood it can be sanded off with a bit of work...maybe 'mould' is not the most accurate description.

the 'exposed' and 'sheltered' views are on the newer shed which was built last year, and over the course of a dry/warm month or so had a total of 5 coats of oil applied..
you can usualy kill black mould with a chlorine cleaner - there are some kitchen spray cleaners that are branded as mildew killers. Once dead it loses its grip and will clean off easily. Might be worth a try on a test area.

OOI, is the oil treatment thick enough to make water bead up and roll off?

I haven't used Tung, but I found with my linseed that you can clean off splashes quickly with White Spirit, but once it oxidises, it sets into a sort of varnish and won't come off.
I'll try the cleaner..

The oil was applied with decreasing amounts of thinning (with white spirit), and the last 2 coats put on 'neat'...initially it seemed to work (water droplets ran off), but about dec last year the black stuff started appearing, and has got progressively worse ever since.
have you got an expansion gap between the board of around 2mm when fully dry [several weeks of sunshine] i suspect the wood is epanding in the only direction it can during the winter

although if you can keep the rain out in the winter the expansion will be less :cry: :cry: ;)
when the shiplap was first put on it was allowed to dry out first. A gap was allowed during installation. This gap closed up during the winter and that is where on one side the 'rounding' occured - the other sides were OK. During the warm spring this year the wood contracted to such an extent that some gaps on the exposed side were 3mm

Yes, I'm convinced that if the oil had worked , and the wood remained waterproof, the expansion wouldn't have been so great on that side.

So that's why I'm looking for a different treatment and a way to be able to apply it given the existing oil.
is there any chance water could be running down the back of the cladding, e.g. from a gutter fault?
Thanks for the advice...

Each shed was clad with a complete batch of vac treated shiplap. Where there is no/little exposure to the weather it has not 'blackened'. The exposed area is South west facing and does catch the majority of the rain etc

I've added a few more pics to give an idea


The left hand side of the greenhouse and shed as you look at them is SW - and the left hand side of the shed is the worst one.

In december when this started to happen I sanded one of the doors when it was dry and varnished it. A horrible finish, but that part didn't 'blacken' whereas the oiled one did (see shed2 pic)
JohnD said:
is there any chance water could be running down the back of the cladding, e.g. from a gutter fault?

No..all guttering is sound and runs off to water butts. The inside of the cladding is unaffected by the 'blackening'
JohnD said:
you can usualy kill black mould with a chlorine cleaner

Tried some chlorine based cleaner (most potent I could find) and it only takes off a small percentage of the 'blackening' - so the sander it is..

but recommendations for a new suitable treatment would still be welcome...


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