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Procedures
and techniques for building plugs will vary from individual
to individual and from shop to shop. For simplicity, this
document will discuss the building of a truck cap plug. Remember,
though, that the plug being discussed for illustrative purposes
could just as well be a boat hull, a bath tub fixture, a recreational
vehicle part or other parts, including multiple-piece units.
Several
of the most commonly used materials in plug building are 3/4
inch birch plywood or other high density wood, 2x4 and 2x6
lumber, and a wide variety of fillers for forming radii and
detail lines. First we'll cover the use of wood and existing
substrates, then we'll cover other materials - plaster and
hemp, foam blocks, and paraffin wax.
The
two most common layout techniques are flat sections reinforced
with lumber and skeleton framing reinforced with exterior
emplacements. Skeleton framing is generally used in the construction
of large boat hulls or decks. A skeleton is constructed to
dimension, and the exterior emplacements are added to the
skeleton. The flat, reinforced method is generally used for
plugs requiring straight, non-wavy areas.
As
a finished unit, a truck cap requires an accurate fit to the
bed sections of different makes and models of trucks. Therefore,
one of the first steps to address is the marriage of the unit
to the truck bed section. After building a reinforced, level,
stationary platform and after marking the dimensions of the
bed of the truck directly on this platform, we can then establish
a line of reference to begin our layout work. This procedure
lends itself well to making recreational vehicle parts that
must fit in rough openings or to boat parts that must fit
over existing parts, for example, a deck plug that goes over
an existing fiberglass hull section.
Now
that we have a system of determining what the outside dimensions
are to be, we can build our rough layout up to these lines
of reference. We are now ready to cut all our straight plywood
pieces and to reinforce them with the 2x4 or 2x6 lumber. Use
kiln dried lumber and exterior grade plywood. By reinforcing
each piece of plywood with lumber placed on edge and not flat,
we can hold the surface of the plywood straight and true.
The
plywood should be cut very precisely with all the edges that
are to become radii cut at a bevel. This bevel cut will allow
the radius to be formed with a filler that can be worked into
shape, which is easier than attempting to form these radii
by shaping the plywood itself with sandpaper or files.
When
attaching the lumber to the plywood, inset all screws or nails
into the face of the plywood. When we finish the exterior
of the plug, we will fill these fastener holes with filler
or wood dowel pins that should be sanded flush. Wood dowel
pins are the better selection, as shrinkage will occur with
most fillers. In addition to screws or nails, use wood glue
to help fasten the lumber to the plywood.
After
the Plug has been totally assembled, except for the radii
and the finish, seal the inside or under side of the wood
with polyester resin or another water-resistant material.
This will prevent warping due to humidity as we continue to
build our Plug.
If
a male plug is being constructed, the outside of the plug
becomes the inside of the mold. Therefore, the gel coat surface
will be on the exterior side of the part. This means that
if the part needs to fit over another object, we will have
to allow for the thickness of the laminate.
When
the design of the plug requires a split mold, in order to
remove the part, the plug should be constructed as a single
unit. The split areas can be determined after the rough forming
is completed.
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