Making Fiber Glass Bodies
by
"Ecurie Martini"

This may really be addressing questions that have already been answered but I love nattering on about history - be forewarned - if it is not your "thing" - skip the rest.

First observation - the idea that fiberglass, glass fibre or GRP (glass reinforced polyester) is unsuitable for body shells on the basis of function, cost or fabrication problems is right there with the aerodynamic proof that bees can't fly on the basis of wing loading calculations! I have a number of 1/24 and 1/32 GRP bodies that I made in the late 50's and early 60's and several Braverman bodies from the same era.

Now, some things have changed. Several of the 1/24 glass bodies were made in molds pulled from resin bodies.  In view of the fact that mold-making was a laborious business in those pre RTV silicone days involving brushing on multiple layers of latex mold material and reinforcing the successive layers with cheese cloth - hours of work. The answer is simple: Resin in those days was weak, heavy and very brittle and a few experiments clearly demonstrated that a body thin enough (after laborious grinding) to be useful would have a very short life - from start to the first real shunt.

Now, GRP is not without its problems. Polyester resin is weak, brittle and stinks while curing. The strength of a GRP laminate is pretty much proportional to the glass/resin ratio - ideally it is almost all glass with just enough resin to wet all the fibres and "glue" them together. This is not hard to achieve in, for example, building 1:1 boats (or even large model yachts - e.g. "A" class) where vacuum bagging or other methods can be used to squeeze out excess resin. (I have never tried this on a small object in a female model - a body shell - but I think it would be messy) The process is at least a two step operation. First, a gel coat has to be laid in and allowed to cure enough to keep the cloth layer* from appearing on the outside of the final molded part and then a second layer of cloth, fully saturated with resin must be bonded to the gel coat. Here is where the problems begin in our small models. If this second laminate containing the reinforcing cloth is not uniformly bonded to the gel coat (i.e. there are voids or bubbles), a hidden flaw will be created. Experience teaches that this hidden flaw almost never manifests itself on demolding or in the early stages of finishing. The weak, unsupported gel coat waits until at least the first finish coat has been applied to collapse, revealing a canyon like flaw. In addition, typical commercial polyester resins (car repair stuff) are adequate but not outstanding in reproducing detail and may not always cure to a hard, paintable surface (they are not formulated to cure in the anaerobic conditions inside a mold - there are special gel coat formulations for that - the only place that I have ever found them in reasonable (model shop) quantities is in kits designed to repair marine gel coats. - but, 40+ years ago, there simply was no alternative.

"Fast forward" XX years - modern two-part resins - they are tough, flexible and excellent at reproducing detail from the current silicone rubber molds. In my experience, they are not as strong as a really well made GRP shell but they are stronger that most injection molded bodies. Because they are typically "slosh cast" in one or more steps or done by a single pour into a two-part mold, the "void" problem that plagues GRP bodies does not occur with any great frequency (bubbles may happen but they are typically on the surface and can be dealt with before a major investment in time has been made.

I like a hybrid approach and have had some luck with the following: I slosh cast a thin layer of two-part resin and allow it to cure. This shell is too thin and weak to be removed from the mold. After it has cured, I coat the inside with a low-viscosity, slow curing epoxy** resin and laminate in a layer or two, depending on the size of the body, of reinforcing fabric - either a very thin carbon fiber or kevlar scrim, and overcoat with the same epoxy to assure saturation. (the issue of voids mentioned above in re GRP can occur but the very light fabric material helps in molding the reinforcement to the interior of the resin shell) This approach can produce a shell which will compare favorably with a vac-form of reasonable thickness ( 0.020") for weight

Good day, ladies and gentlemen. There will be a quiz on this material next Friday.

EM

* the glass cloth in a typical automotive patching kit is really too thick for our purposes - glass fabric is available in all sorts of weights - I have used a material that is about the weight and weave of shirting fabric

** In addition to forming stronger bonds, epoxy resins typically retain some flexibility on curing in contrast to the glass-like brittleness of polyester.