Fiberglass over balsa planking

by N. Carlson
(Salem, Utah)

What is the best technique for applying fiberglass over a balsa planked hull on a model tug two feet long?

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The best way to fiberglass a model boat hull may depend on your workspace, experience and budget. Here is a short "primer" for anyone interested in working with fiberglass and Epoxy/Polyester resin.

Materials: Resin and Fiberglass

For our purposes there are essentially two kinds of resins to consider: Epoxy and Polyester. Both are two-part compounds. Epoxy consists of a Base (part A) and a Hardener (Part B). The components that make up Polyester is usually referred to as Resin and Catalyst.

You should be looking for Lay-up Epoxy. It has a very light consistency compared with Epoxy glue that most are familiar with. The light viscosity is essential to wet the fiberglass.

If Epoxy is mixed in incorrect proportions, it can spell disaster. Too much Hardener and the resulting laminate is weak and brittle. Not enough and it won’t cure. Instead it will stay tacky for weeks and never develop any sort of hardness.

Controlling open-time (time before it cures) and pot-life (effective time to work with the resin) with Epoxy can be tricky. Many manufacturers offer several options of Hardeners that gives different open-times. Temperature plays a large role – lower ambient temperature leads to longer cure times.

Often a Hardener that provides a long open-time will cure tacky in humid weather. So that is one other thing to consider.

Polyester in that regard is much more forgiving. The catalyst is just there to accelerate the crystallization that is technically already taking place, only really slowly.

So the speed at which polyester cures can be controlled by the amount of catalyst added.

Polyester resin has a much shorter shelf-life. I seem to recall you should use it within a year (I could be wrong, check with manufacturers).

Here is a short comparison between the two:

Epoxy

• Non-toxic when ingested or fumes inhaled – can ship by air.


• Non-flammable


• Little or no odor


• Expensive


• Critical mixture ratio for optimal strength and proper cure


• Almost infinite shelf life


• Difficult to control pot-life and open-time


• Repeated contact with the hardener will lead to sensitization


• Will not dissolve thermoplastics such as polystyrene foam

Polyester

• Volatile fumes – can only ship ground


• Flammable


• Strong and nauseating smell


• Relatively inexpensive


• Mixture ratio non-critical


• One year shelf life


• Change mix ratio between Resin and Catalyst for infinitely variable open-time


• No long-term health effects as long as a proper respirator is used


• Will dissolve thermoplastics and most foams

Which to choose? Well, if you can only work in your house or in an attached garage or basement: it’s Epoxy. The fumes from the Polyester Catalyst is so permeating that there is no way to work with it under those circumstances. Even your clothes will stink.

I wouldn’t recommend working outdoors, but it’s an option where Polyester may work out.

My shop is in the basement, so I'm strictly using Epoxy.

Fiberglass and other cloth

Most common fiber mat materials are Fiberglass, Carbon Fiber (CF) and Aramid (Kevlar). For those building competition boats, either RC sailboats or power boats CF and Kevlar can offer advantages. For the rest of us, we’re best off sticking with Fiberglass.

Fiberglass come in different qualities, textures, weaves etc. It can seem like a jungle, but here is a quick rundown of what some of the terms mean:

Fiberglass Cloth

Fiberglass Cloth comes in a multitude of weights. The weight refers to actual weight of a square yard. The unit of measure is oz. So you’ll find 0.6, 0.7, 1.4oz. etc.

For covering planked model boats, the lightest weights work the best. They drape much better and therefore are a lot easier to work with. The down-side is that they are easy to sand through. Sometimes more that one layer is necessary.

Don’t get confused with terms like E-glass, S-glass, Hybrid Fabrics, Twill weave, unidirectional fabrics etc. For our needs, the simple stuff will work just fine.


Fiberglass Mat

Fiberglass Mat is unsuitable for covering the outside of a model.

Fiberglass Mat is a non-woven material. It is loose fibers that are pressed and held together with a thermoplastic binder. The idea is that when the Mat is soaked with Polyester, the solvent will loosen the fibers from the mat but stay within the applied Resin. This is what you’ll see on the inside of most molded Fiberglass items. It is easily recognized by the random orientation of the fibers.

It is the best solution if you make your own fiberglass hulls from a mold. Since there is a dissolve-phase, this mat will only work with Polyester – not with Epoxy. The Mat material comes in different weights. The Heavier weights are stiffer and thus harder to work with and more difficult to take tight radii.

Safety & Hygene

Chemicals such as Epoxy and Polyester Resin should be handled with care. The more you know of the health risks, the better chance you have to protect yourself and those around you.

I’m making no claims to cover all aspects, so make sure you inform yourself of proper safety precautions. Ask for MSDS sheets – all manufacturers are required to provide one for free upon request.

Here are a couple of things I’ve learned over the years. Not to be complete, but this is what I go by:

Epoxy
Protect your hands and avoid direct contact at all cost. I’ve been told latex doesn’t work – the Epoxy bleeds through. A better option is to use a barrier cream and either a vinyl or nitrite, neoprene or nitrile glove. Unless you have developed sensitization, a respirator should not be necessary for occasional use.

Polyester
Neoprene, Nitrite and I believe Latex will work. Again, I’d use a barrier cream, just to be sure. Vinyl won’t work; instead they will dissolve either from the Resin or acetone used for clean-up.

Respirator is a must. I’m not talking about a dust mask here, but a real face mask with chemical vapor filter cartridges. These are the same as you’d use for professional spray painting.

Other things, avoid working in shorts and flip-flops. Spills happen easily and large amounts directly on your skin can be difficult to clean up.

Work Procedure

It’s not that easy to generalize and say this is how it’s done – there are always so many exceptions. Experience will tell you when to take what path. Having said that, and assuming the experience level is modest here is how I’d do if I hadn’t done it before.

I’ve never worked with Polyester, but the method is very similar.

• Sand the hull smooth and fill all cavities and holes with Bondo or similar. I’d typically go to about 18-grit sand paper. Don’t prime or use any type of base coat. It’s unnecessary and will only get in the way.


• For a beginner it’s easiest to use a patch-work approach to applying the fiber glass cloth. There are several advantages. The idea is to lay single strips from keel to railing vertically, but short segments along the length. The curvier the hull, the narrower the strips (more segments along the length).


• Cut 30-50% more strips than you think you’ll use.


• Starting at the stem and working towards the stern. Alternating left and right.


• At the keel, overlap the strips over the keel.


• Take a paint brush and cut the bristles about half-way to create a pretty stiff brush. It should be an inexpensive brush, but of appropriate quality – the bristles should not fall out as they tend to do on the least expensive Chinese and Indian brushes. Don’t spend a fortune either. After the Epoxy cures, the brush is dead.


• Mix the Epoxy and work as fast as you can.


• Position the fiberglass with one hand and wet it with the brush in the other hand. Start in the middle of the strip and wet it out towards the edges. You have to use a stabbing motion, not sweeping as you would when painting. If you sweep (especially at the edges) the cloth will fray and look ugly.


• With the brush, work out any air bubbles that may appear. Usually repeated stabbing will make the air disappear through the cloth.


• Keep covering. If you run out of Epoxy somewhere mid-way, try and mix up a new batch quickly and keep going.


• Any wrinkles need to be worked out. Sometimes they will come out easily by carefully working them with the brush. Sometimes you’ll have to lift the cloth and tug on it a little.


• Once covered, focus on the edges – get bubbles and air gaps out.


• Let it cure, but come back within a few hours and assess how much clean-up you’ll need to do


• As soon as the hull can be handled (usually after 24 hours for slow curing Epoxy), do a rough trim of excess glass cloth and epoxy along shear line or bulwark.


• At some later time depending on cure speed, start sanding. Go easy on the hull itself, but clean up the edges and bumps. Worst case you’ll need a second or third layer.


• To lay subsequent layers, the hull needs to be cleaned from the fatty residual layer that forms on top of cured Epoxy. Iso-Propyl Alcohol works well for this.

Material and Vendors

The Epoxy I use is West System by The Gougeon Brothers. I use the 105 Base and the 206 Hardener. Many local boat supply places sell it or you can order online.

Fiberglass Cloth can be bought from www.cstsales.com.

Hopefully that will get you started. Best of luck with your model boat project!

Petter