On Mon, 26 Mar 2007 17:50:22 +1000, Peter Hendra
wrote:

On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman"
wrote:



A friend has gone thru this with his stainless. People at his workplace
that know about his sort of thing told him he needed to passivate the welds.
They gave him some solution to paint on the welds. I believe this is
hydrofluoric acid, perhaps a mixture with other acids. Requires very
carefull application. It took away the surface shine. Don't recall the
final result but I can ask him.


Thanks for this Gordon,
At the moment the offending articles are sheathed by masking tape as I
am halfway through spray painting the hull.

I would very much appreciate you asking your friend if it is not too
much trouble. The more information about this, the better.

When it was welded, it, like all other Stainless welds was brushed
with that nitric acid gel solution that then is washed off with water,
and which removes the discolouration from the tig/mig/arc welding
process. Is this the same thing? I can't recall getting anything
"passivated".

Thanks and cheers
Peter

The acid treatment was "passivating".

My experience is that unless the stainless is polished to a mirror
bright finish you will get staining. All the bits have to be polished;
Assuming that it is a bolt on fitting the bolts and washers that are
exposed to sea water need to be polished.

I've been making things out of stainless and putting them on boats for
quite a number of years and I have found that if the part has all the
welds ground smooth (so the welding ripples don't show), no pin holes
or voids in the weld and then polished bright I seldom get staining.
If I don't do this I always get staining.

A couple of years ago I built a new stitch and glue dinghy. Welded up
a couple of plates with a lifting eye; dosed them with a 20% acid bath
until all the discoloration was cleaned off; gave them a lick with a
sanding disk on a high speed grinder; mounted them and gave them two
coats of epoxy primer and two coats of two part poly urethane
(spelling?). Got "rust" stains. Should have polished them.

By the way, I get my stainless polished at the local chrome shop,
Cheaper then the "stainless shop".

I also take care to bed everything with either 3M or Sikaflex. Much
better then household "silicone" sealant.

I've been reading some of your other posts -- do you reckon that your
paint job is cheaper, or more expensive then it wold be in Malaysia?



Bruce in Bangkok
(brucepaigeatgmaildotcom)

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