On Jun 2, 7:42 am, Stealth Pilot
wrote:

when you do a conversion what do you do to get all the oxide off the
crankcase?
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Trick question, right? (Most conversions begin with a new crankcase.
No corrosion.)

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the case I'm looking at converting is oxided a dark grey.
I'd like to get it all clean again before painting it. ...without
stuffing it up.

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Dark gray may not be bad. (It depends on the corroding elements.) A
good de-greasing followed by a good scrubbing with a detergent having
a neural pH, followed by a boiling-water rinse, may be all you need
before painting.

A wash with dilute phosphoric acid, ideally with chromate extenders,
will brighten the surface. It produces a black phosphate residue in
doing so that must be removed. You won't see the black stuff if you
scrub-on the phosphoric with a paint brush cut down to make the
bristles stiffer, but it will show up if you just allow the thing to
sit.

But if there is any corrosion due to salts you'll probably have to
boil the case to get rid of the salt before you can address the
corrosion.

Worse-Case: (Old, salvaged bus engine.) Degrease, wash, then blast
with crushed walnut shells.

There are commercial products available specifically for prepping
magnesium alloys. Some customers include the surface-prep & finish
specs in their order. But some of that stuff costs the earth and I
frankly haven't seen much difference compared to the methods I use.

I'm not a chemist but the dividing line would seem to be between
simple oxidation of the surface, which does not alter its heat-flow
properties and which may be painted without further prep, and
destructive corrosion that etches the parent metal and produces
various crusty-looking residues that act as thermal insulators.
Removing the SYMPTOMS of such corrosion does not address the cause and
painting over the apparently clean surface ensures the corrosion will
re-appear.

I know that sounds sorta wishy-washy but there is some overlap between
the types of 'corrosion.' On old/used crankcases I usually do a few
tests before deciding which surface-prep procedure to follow.

Since paint itself acts as an insulator, use the thinnest coat
possible. And NEVER use any 'high temperature' paint, such as that
advertised for wood stoves, barbeques, exhaust pipes or what-have-
you. Such paints get their hi-temp qualities from clays or metallic
salts and are excellent thermal insulators.


-R.S.Hoover

PS -- A boat yard can usually point you toward products containing
phosphoric acid meant to prep non-ferrous alloys for paint. Ditto for
a good paint store that deals with aluminum siding and window
screens.