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Re: Source of Wheels and Paint Question

-----Original Message-----
From: Fred L. DeRoos <flderoos@mmmpcc.org>
Date: Sunday, April 19, 1998 4:21 PM
Subject: Source of Wheels and Paint Question

>Also, has anyone repainted the area directly above the rear bumper?  I'm
>finally starting to get rusty areas that look like they are where the
>bumper or chrome strip attaches to the body (regular spaced spots of
>rust directly above the chrome).  What do you do about the galvanizing?
>I can grind it down to bare metal, prime it and paint it, but will that
>last?  I assume that even if a body shop did it, they wouldn't
>regalvanize it.  Any suggestions??

Hi Fred;

    Ah yes, the old "rust stains avove the rear bumper chrome strip"
problem. I've been fighting this one on both my dearly departed '86 5ktq and
my '91 200q. The liberal amount of road salt the DOT uses in the winter
accelerates this problem nicely.
    What appears to happen is the chrome trim strip mounting bosses cut
through the paint where they contact the bodywork. This usually happens at
the mounting bolts, but will happen elsewhere if the trim strip is warped.
Galvanic action between the chrome and galvanized body steel accelerates the
corrosion. I have tried several fixes and my last attempt appears to be
holding up on the '91 200q.
    Remove the bumper and the chrome trim strips. Sandblast the corroded
areas back to clean galvanizing; the zinc is easily discernable by it's
"paler" colour. I recommend sandblasting over grinding because it removes
all the corrosion whereas grinding can leave corrosion in small pits which
are lower than the ground surface. Sandblasting also conserves more metal
and galvanizing and provides a good mechanical key for the paint.
    Feather the sandblasted areas with 400 grit wet paper. Chemically treat
the bare metal with an acid-based metal prep (such as Dupont M3 metal
conditioner); this cleans and passivates the metal and chemically etches it
for better paint adhesion.
    Prime with an epoxy wash primer followed by a good two-part polyurethane
primer. Topcoat and go heavy on the clearcoat in these areas.
    This just gets you back to where you were before and the problem will
re-occur if you just re-assemble everything. What I have done is cut plastic
washers large enough to cover the trim strip mounting bosses ( I used
polyethylene from a gray antifreeze container which nicely matched the
titanium gray paint!). Seal the plastic washers to the body with automotive
dumdum, trim off the excess dumdum, and moint the trim strip.
    I tried using just the washers, and the corrosion eventually came back.
The dumdum seal appears to have solved the problem (at least to this point).


Fred Munro
'91 200q  250k km