Probability is that the welds are not ground flat and smooth and
crevice corrosion is underway in the 'intersticies' of the weldment
(poor weld technique ... not a full penetration weld).

To be 'corrosion resistant' (chlorides, etc. attack stainless) all the
weld must be flat /smooth by grinding and then polishing/buffing to a
high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld
will be a source of 'rust' .... gotta be closed and then smoothed. .


In article , Peter Hendra
wrote:

Hi,
I know that this possibly has a simple answer that everyone but me
knows, but is there any way I can prevent the unslightly brown "rust"
marks on my white hull paint from a welded fabricated stainless steel
fitting that is bolted on with stainless bolts?

I am about to spray on the white topcoat over the primer during a hull
repaint job and I'd like it to stay looking good.

I suspect that the drop ladder on the stern is not made entirely of
316. It has been 15 years since I fitted it, so that I can't recall
whether the bolt is 316, 304 or (unlikely) something else. There are
no 316 markings on the bolt heads.

We use oxalic acid powder mixed with water to get rid of it but it
always comes back. I am beginning to quote Lady MacBeth when I rub it
away ("out foul spot") and it's embarrassing when passersby overhear.
I know that there are several (expensive) "special" products to clean
it with but I would appreciate some knight on a white charger offering
me a permanent fix. It doesn't matter if he's young or old, handsome
or otherwise - I'm not playing Sleeping Beauty - so long as he or she
has the answer.

I wonder whether it would be possible to electrically isolate it with
say, a teflon washer?

cheers
Peter
(who actually sails places and doesn't pretend to)