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?

--
Cheers
Dave.

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