FIBERGLASSING FOR MODELERS
(Part 3)
By Cecil Collum
There are many more materials that can and have been used for laminates and construction but are of little use
in modeling airplanes. Fillets, fairings and gluing can be done with both of the primary systems which are used
in modeling: polyester or epoxy resins. By adding various fillers to these resins, one may make fairing and
fillets fairly easily.
There are essentially three types of filler/ additives for resins:
The first is fumed silica used to thicken the mix and add thixotropic properties to allow the mixture to hang on
vertical surfaces.
Secondly, there are bulking agents which reduce the density of the putty and make it easy to sand, including
micro-balloons, microspheres and wood flour. Micro-balloons are microscopically small glass or resin spheres which
add light weight bulk to the mix and add structural strength.
The third are milled fibers which add a matrix of glass fibers to the mix for added structural strength.
It is now time to discuss Laminating, a basic skill and involves adhering a laminating material over a substrate
of wood, foam, or some other material. Wood being the most common substrate to which we adhere the cloth, we need to
have a basic understanding of it. Wood is composed of cells and for a laminate to adhere properly, we must penetrate
those cells with resin as much as possible. To do this, we must apply a "pre-coat".
When using polyester resins, the pre-coat is made by properly catalyzing a batch of resin, allowing it to sit for
a moment to get the reaction going then reducing or thinning it with 10% acetone by volume. Then paint or roll the
mixture on the wood and allow it to cure for an hour or so. Pre-coating acts like thousands of tiny resin nails
penetrating the wood and increasing the bond of the layers of resin and cloth to be added next.
Pre-coating with Epoxy resins is basically the same EXCEPT thinning with anhydrous (no water) alcohol. Once the
wood is pre-coated unthinned resin can be brushed on the surface and material applied. Foams and other "core"
materials will need a pre-coat before applying material or attempting to bond a second core to the first.
There are many more materials that can and have been used for laminates and construction but are of little use in modeling airplanes. Fillets, fairings and gluing can be done with both of the primary systems which are used in modeling: polyester or epoxy resins. By adding various fillers to these resins, one may make fairing and fillets fairly easily.
There are essentially three types of filler/ additives for resins:
-
The first is fumed silica used to thicken the mix and add thixotropic properties to allow the mixture to hang on
vertical surfaces.
Secondly, there are bulking agents which reduce the density of the putty and make it easy to sand, including micro-balloons, microspheres and wood flour. Micro-balloons are microscopically small glass or resin spheres which add light weight bulk to the mix and add structural strength.
The third are milled fibers which add a matrix of glass fibers to the mix for added structural strength.
It is now time to discuss Laminating, a basic skill and involves adhering a laminating material over a substrate of wood, foam, or some other material. Wood being the most common substrate to which we adhere the cloth, we need to have a basic understanding of it. Wood is composed of cells and for a laminate to adhere properly, we must penetrate those cells with resin as much as possible. To do this, we must apply a "pre-coat".
When using polyester resins, the pre-coat is made by properly catalyzing a batch of resin, allowing it to sit for a moment to get the reaction going then reducing or thinning it with 10% acetone by volume. Then paint or roll the mixture on the wood and allow it to cure for an hour or so. Pre-coating acts like thousands of tiny resin nails penetrating the wood and increasing the bond of the layers of resin and cloth to be added next.
Pre-coating with Epoxy resins is basically the same EXCEPT thinning with anhydrous (no water) alcohol. Once the wood is pre-coated unthinned resin can be brushed on the surface and material applied. Foams and other "core" materials will need a pre-coat before applying material or attempting to bond a second core to the first.
The best bond between two pieces of wood can be made by first pre-coating both pieces with epoxy, then a layer of resin applied, then press the two together and allow to cure. A laminate is any piece of material that has been impregnated with resin. Most laminates are more than one layer of either material, or material and cores.
The material or cores in a laminate contribute most of the strength and characteristics, while the resin bonds the layers together and transfers the load from layer to layer. As a rule of thumb, use only enough resin to "wet out" the material, as excessive resin only adds weight and actually weakens it. The material weave should be clearly visible when the laminate cures. Slick or obviously resin rich areas are weak spots.
One should plan to use the same weight of resin as material, or, in other words, 6 ounces of material requires 6 ounces of resin to properly wet it out. Many of the cowls now are made of fiberglass with a gel coat, a Polyester resin, on the exterior which means that eventually repairs will be necessary.
Few of us have made repairs to a gel coat and shy away from it as long as possible, so below we will cover the basics of gel coat repair:
(Note: One can use Bondo or similiar systems over Polyester but not over epoxy. The Bondo will simply roll off epoxy as you try to sand it).
- The basic sequence for repairing a gel coat is to first excavate the site with sandpaper or small grinder
in a Dremel tool. These tools rotate extremely rapidly, so be careful! Round all edges to eliminate stress and
clean the site with Acetone.
Tape around the site to allow for an overfilled spot due to the Polyester resin shrinking. Fill the hole with properly catalyzed gel coat or patch paste and GO AWAY! Don't check it every few minutes! Leave it overnight if possible, then the next day you can remove the tape and begin wet sanding with 220 or 320 wet or dry sandpaper, working up to 2000 grit.
Complete the repair by polishing it with a carnauba based wax, which protects the new gel coat.