Rubber Bumper Repair Basics

Rubber Bumper Repair
Basics
by Lars Grimsrud & Butch Powers
SVE Automotive
Restoration
Musclecar, Collector & Exotic Auto Repair &
Restoration
Broomfield, CO
This tech paper will discuss
basic repairs to the rubber & plastic bumper facias typically used by GM
and other automakers. These rubberized components have been around since
the GTO “Endura” bumpers of 1968. The parts often erode,
tear, or get gouged, but can be easily repaired to like-new condition if
the correct processes and materials are used. Different auto body
professionals have different preferences as to how to do things, and I may
get some disagreement on the specifics of this article. This article will
provide you with a group of products and materials, and my personal
technique, for accomplishing first-class results.
General
Info
The rubber bumper facias used on Corvette front and rear bumpers
actually hold up pretty good. The most common problem is rapid
deterioration of the factory-applied clearcoat on these components.
Usually, the parts only need to be wet-sanded, properly prepped, and
repainted with the correct materials. In cases where the parts are torn
or damaged, the repairs are very simple, and produce excellent results.
The most difficult and time-consuming part of the job will be removal of
the parts from the car: it is very difficult to get excellent results
without removing the bumpers and disassembling them down to their
component part level, since the tail light lenses on a C4 cannot be
removed from the outside of the vehicle (very easily). Masking the parts
off without removing them does not produce acceptable
results.
Supplies, Fillers, Primer & Paint
In addition to some
of the basic tools and supplies outlined in my “Corvette Body &
Paint Repair Basics” article, you will need the following
“stuff:”
1 qt Transtar Hydroflex Waterborne Acrylic
Primer, P/N 1234
1 kit SEM Rigid Sem-Weld II, P/N 39508
1 qt DBU
Basecoat in the correct color
1 qt DRR1170 basecoat activated
reducer
1 qt Concept 2020 clearcoat
1 pt DU4 hardener
1 qt DT870
reducer
1 pt DX369 Flex Additive
Econo-grade lacquer thinner for
cleanup
150 grit sandpaper
400 grit wet-or-dry sandpaper
600
grit wet-or-dry sandpaper
Masking tape
Paint strainer
Rubber
sanding block
Paint gun & compressor
Technique
Gouged,
torn & damaged areas:
NOTE: Before doing any work on any parts, be
sure to degrease them well with a good wax & grease remover. If silicone
products have been used on the car (like Armour-All on the tires), use a
good silicone remover and wipe the parts down well. If you don’t do
this, you will grind silicone and contaminants into the rubber parts
during the sanding process, and your paint will “fisheye” like
crazy.
1. Tears and cracks in the facia should first be
“stop drilled:” at the very end of the tear or crack, drill
an 1/8″-or-so diameter hole all they way through the part.. This
will relieve the stress at the end of the crack and prevent it from
propagating any further.
2. Using a grinding wheel, cutoff wheel,
Dremel Tool, or other suitable equipment, prepare the crack, tear or gouge
by ‘V-Grooving” the damaged area and removing all loose
material. If the material is thick enough, V-groove both the front and
the back side. If not, at least V-groove the front (top) side.
It’s important that the filler material you’ll be applying has
a good, fresh groove to be squeezed into in order to promote good bonding
adhesion.
3. Fabricate a backup strip. If the tear or crack is in
a particularly flimsy area, cut a thin piece of metal or plastic to bond
onto the back side of the repair to stiffen it up.
4. Mix up a
little bit of the Sem-Weld II. Apply it to the damaged area, and bond
your back-up strip to the back of the repair using the SEM (if needed).
Squeeze the SEM really good into the damaged area.
5. Once cured,
sand and shape the filled area with your 150 grit. Apply a second coat of
SEM if needed, and re-sand.
Part is repaired and ready for
refinishing!
General Refinishing
Most of the rubber facias are
not damaged, and are only oxidized and/or slightly crazed (like their
owners). Here are the steps to refinish these parts, as well as to finish
off the parts that have been repaired following the repair procedure
above:
1. Wet sand the entire part with 400 grit and a sanding
block to remove all oxidized and loose paint. Sand all crazing smooth and
blend out any chips in the paint with the surrounding areas. Sand the
repaired areas described above with the 400. Dry the parts
well.
2. Shoot 2 coats of the Transtar Hydroflex Waterborne
Acrylic Primer onto the parts. This thick, black, rubbery primer does not
need to be reduced, and will clean up with water.
3. Let the parts
cure overnight. If you attempt to sand the parts any sooner (in spite of
what it says on the can), the primer will ball up and peel off the
parts.
4. Wet sand the parts with 400 grit wet on your rubber
sanding block until smooth. If all of the defects are completely gone,
switch to 600 grit, and sand it again. If there are still a few little
imperfections, dry the parts off well and lay another coat of the
Hydroflex onto the parts. Wait a day and finish the parts off with the
600 grit wet.
5. The parts can now be topcoated with your basecoat
color, mixed 1:1.5 with the activator.
6. After 30 minutes, mix
your 2020 clearcoat, but mix it 4 parts 2020 to 2 parts DT reducer to 2
parts DU hardener to 1 part DX369 flex additive. This will keep your
parts from cracking and crazing. Give the parts a dry tack-coat, and then
follow with a couple of nice, wet coats.
7. If desired, the parts
can be “color-sanded” after a day with 1500 grit and buffed
out as I described in my Paint Basics article. If you leave them in their
as-painted condition, they will look very much like the new, factory
finish.
Other Notes
The only problem I have run into during this
process has been when shooting some of the more translucent basecoat
colors: metallic red, for some reason, is fairly translucent. The
underlaying black Hydroflex primer will make the translucent colors look
darker than they should, unless you put a LOT of coats of basecoat on the
parts. Generally, heavy basecoat buildup will degrade the longevity of
your paint job, so this is not desireable. To avoid this when working
with the more translucent colors, I have been giving the primed rubber
parts a light coat of white basecoat before applying the actual basecoat
color. You can also shoot a light coat of your grey or white DP epoxy
primer onto the parts to achieve the same effect. This will assure you of
a proper color match with no bleed-through of the dark base
surface.