On 16/06/2020 22:48, petek wrote:
On Tuesday, June 16, 2020 at 9:30:11 PM UTC+1, Dave Liquorice wrote:
On Tue, 16 Jun 2020 09:12:32 -0700 (PDT), petek wrote:

I have used some zinc yellow passivated screws on my gates to fix
the T&G facing boards to the gate structure.

Steel of any form is useless outdoors. Brass or stainless only,
brass by preferance, stainless is a bit brittle.

I need 4 x 45mm countersunk, posidrive preferably.

Toolstation list 1 3/4" No 8 slotted brass, £8.84/200,
unfortunately OOS for collection or delivery.

B&Q list 4 x 40 £5.37 or 4 x 50 £5.65 brass pozi packs of 25.

What will be the effect, in the short term and long term, of
leaving the screws in place and just going ahead with the
Sadolin?

Irespective of the Sadolin they'll rust... Does iroko really need
treatment anyway?

Thanks to both Dave's for helpful comments. Yes, Iroko is a very
durable hardwood (similar to Teak) but turns a grey colour with age.
I want to retain the natural golden brown colour, hence the Sadolin
treatment. I don't really understand why steel screws would be
susceptible to rust as the length of the screw would be protected
within the timber (Iroko is naturally oily) and only the top of the
head exposed to air and water. Hence my question about short & long
term effect. I thought there may be some possible discolouration
effect similar to steel screws in oak, or perhaps some other obscure
chemical reaction would affect the screws or timber. If just the top
of the head turns a bit rusty, it wouldn't be too noticeable and I
could possibly live with that. They're zinc passivated so maybe that
would hinder corrosion a bit. Anyway, to be on the safe side, I'm
going to get some 4 x 40mm brass from B&Q tomorrow in the hope
they'll be long enough. The gate frame is 30mm thick so that leaves
only 10mm to enter the facing boards which themselves are 18mm thick,
but maybe with a deep countersink that will be OK. If that fails I
like the idea of a drop of clear lacquer on the head of each screw.
Cheers Pete

Iroko is almost as good as opepe in terms of surviving in the weather.
Unless the screws are stainless steel they will rust and expand which
will damage the wood. These oily hardwoods are brittle so it is worth
using either stainless or brass fittings.

They all turn grey eventually due to UV exposure but the weathering is
only skin deep. I would seriously use an external use oil on it rather
than modern water formulation of Sadolin which IME tends to be somewhat
opaque and flakes off oily hardwood in a messy fashion after a few years
outside - at least that it what it did on our outdoors VH furniture.

The best outdoor system I found was some spirit based Dutch stuff from a
Dulux decorator centre - I forget the exact name. It was expensive but
it was also the only one that really stood up to continuous outdoor
exposure really well. It doesn't seem to be available any more

But they do still have a decent range of outdoor oil based stuff.

https://www.duluxdecoratorcentre.co....ls-preservers/

--
Regards,
Martin Brown