>
> As a recent purchaser of a GMC M/H, I'm wondering what
> is the roof made of. I had always "assumed" that the
> shell (top and sides) were sheet aluminum with cast
> fiberglass end caps. Am I wrong here?
>
>
> * D C "Mac" Macdonald *
> ** Oklahoma City, OK **
> * "Money Pit" 76 x-PB *
> _______________________________________________
You are pretty close. The center section of the roof is aluminum as
are the upper sides above the beltline.
The rest is a form of fiberglas that is called SMC.
Here is an article that I wrote for the GMCMI newsletter issue 96,
Summer 2006, that might be of some interest to you.
GMC FIBERGLASS REPAIR
The earliest 1973 vehicles have “hand laid”fiberglass, at least
for the front body assembly. Somewhere between serial number 24 and
50 this process became SMC (sheet molded compound). This was a new
composite material for use in the automotive, industrial, and personal
watercraft markets that presents unique repair problems. Parts made
with SMC are produced in compression molds, so they are smooth on both
the inside and outside. SMC parts do not have an outer gel coat, but
they are usually painted or color molded. When the paint is sanded
off, the underlying surface has a marble appearance. When damaged SMC
is sanded, short coarse fibers are exposed and a dryer powdery dust is
produced compared to conventional
materials.SMC is a polyester-based
material, but it cannot be repaired with polyester resin. This is due
to the mold release agent that is present throughout the entire SMC
part. Unlike conventionally molded parts where release agents are
applied to the mold surface, SMC is compounded with them in the resin
mix for quicker processing. This means that as the damage is sanded to
prepare a good bonding surface, fresh mold release agent is exposed.
Polyester resin products are not strong enough to adhere to this
surface. SMC SHOULD ONLY BE REPAIRED USING EPOXY-BASED RESINS, FILLERS
AND ADHESIVES. For good adhesion, when painting, use only catalyzed
type paint systems -- at least for the
primer.When a part is broken or
crushed, it is difficult to realign the pieces with each other because
the frayed fibers tend to “hang up” on one another. Use a saw
blade to cut the length of cracks or tears. This relieves the stress
on solid laminates which often return to their original shape with
little force. A router is excellent for removing damaged core
material without disturbing intact face skins. Try to remove as little
material as possible so the repair does not grow too large. However,
solid laminate must be exposed for a good
repair.Next, support the
part so nothing gets distorted during the repair process. This can be
as simple as strips of 2 inch wide masking tape or as elaborate as a
custom-made clamping fixture. With the part supported, proceed to
preparing the bonding surface by grinding a taper or steps around the
damage. This is the critical step for functional repairs but it is
also the most overlooked and abused. If a taper is to be used,
measure the depth of the valley and calculate how far the sanding must
extend to achieve the desired ratio. Mark the outer edge of the taper
using a marker and begin sanding inward toward the valley. Be sure to
remove material slowly so the taper progresses evenly. Wet the entire
bonding area with resin, then begin saturating each ply of
reinforcement before it is placed. Work on a sheet of plastic so the
fabric can be easily lifted from the table once it is ready. A
flexible rubber squeegee is the best tool for spreading the resin
evenly through the fabric and removing excess which may be present.
Place the reinforcement into its spot on the repair, ensuring the
proper orientation. Stop every few layers to compact the patch as much
as possible. A squeegee or grooved roller works well for this.
Continue stacking the repair plies at right angles until all the
material that was removed has been replaced. A final cover layer is
then added over the entire area. Sand before painting.
Emery Stora
77 Kingsley
Santa Fe, NM
