On Tue, 16 Jun 2020 14:48:06 -0700, 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?

--
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

Counterbore the holes and fill them with putty.