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?

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:

  • 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

  • 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:

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.

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

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


Comments for Fiberglass over balsa planking

Click here to add your own comments

Polyester resin and epoxy incompatibility
by: Martin Schneider

Epoxy will keep polyester resin from hardening, even if the epoxy has time to harden. I put the epoxy on the inside of a balsa hull and let it harden and then tried to apply some polyester resin. I am not going to try and explain my illogical reason for doing that. The polyester never got hard. Apparently the epoxy destroyed the catalyst. I put it in our kitchen oven 1/2 at a time (had to leave the door open). That was successful. It seems like you could place epoxy over cured polyester but not polyester over epoxy

Mold surface preparation
by: Petter

It's hard to say for sure, but I think either path would work. The key is to work the plug to a "Class A" surface finish, and the subsequent mold preparations. As you probably know, this requires patience and a lot of sanding with finer and finer grit sand paper, and finishing with polishing compound and a lot of elbow grease. If going the fiber glass route, there are benefits in using alternating resins for each molding process step. What I mean is, the fact that epoxy and polyester both stick to itself better than each other, you can use that to your advantage by molding each step alternating resins. Also, consider one-piece vs. split mold. Split molds are more difficult to make, but are a lot easier to extract the hulls from. They also last longer as there is less wear when demolding each hull.

Making a mold from a model
by: Dave

I've been searching high and low for advice on how to prepare the surface of my model (the plug) to pull a mold from it. Its a 36" model 46' Hatteras, mostly hardwood plank on frame with some birch plywood at flat areas and faired with automotive Bondo and loads of primer. What I am looking for is advice on how to prepare the laminating surface that is presently just paint. I'm afraid this laminating surface isn't adequately strong to pull a molding surface from. That is, the model is presently the plug I am pulling a mold from.

Should I laminate the model hull with a fiberglass mating surface or is my painted wood model adequate after finish painting with a gloss paint and preparing for mold release in the usual manner?

Good ideas too.
by: Petter

Holding against the light and using a marker makes a lot of sense too. I like the idea of testing in a small area first too. You're on the right track for sure.

by: Olof

Thanks again, Petter!
Yes, I had my doubts about painting, too. So far I have also tried to hold the hull against the light, to get a better view of dips and bumps, and then mark them with a marking pen. It is a long and wide hull (110 x 34 cm:s), so there is a lot to do...
I will test staining on another, similar area first, before applying to the hull.
(The hull is built from 2,5 mm thick and 5 mm wide pine strips and then more or less covered with 2-component filler. :-))


Epoxy & fiberglass
by: Petter

I've never tried it. Something about it doesn't seem right as you want the epoxy to bond to the wood. Painting in between creates a barrier that bonds to the wood. The epoxy bond rely on the mechanical strength of the paint.

A better option would be to stain the hull. A stain is really nothing more than a color pigment suspended in a solvent that will evaporate. It may not work as well since some fillers will not readily accept the pigment.

What I do in these situations is to run my hands over the surface, one hand on either side of the hull simultaneously. With a little practice it is amazing how you'll be able to pick up bumps and dips and asymmetry in general. Don't worry so much about the dips yet. They can be filled over the epoxy (after proper sanding and surface prep). Bumps are the bigger problem as you have to sand through the fiberglass to fix. To be fair, even that is doable, it's just not all that fun and better if it can be avoided.

Hope that helps.

Covering with fiber glass
by: Olof

Many thanks, Mike John!
I have filled and sanded the hull so it´s like a patchwork of different colours. In order to find any unevenness, could one paint the hull in a dark, glossy paint before covering with fibreglass, which in turn will be painted? Or would that be a bad idea?
Thanks again.

Surface smoothness?
by: Olof

Thanks, Petter, that was very helpful! :-)

Pretty Smooth...
by: Petter

It has to be smooth. Fiberglass & Epoxy, as I see it, is part of finishing. What I mean is, all your foundation work with sanding and filling over and over should be completed before the Epoxy goes on. This is not the time to cut corners. The sanding that takes place after Epoxy is mainly to even out any minor imperfections and to prepare for paint. This is usually done with 120 grit and finer sandpaper. I usually go with 120 to 240 and finish lastly with a Scotchbrite pad.

Hope that helps, and good luck!

Surface smoothness?
by: Olof

Hi, thanks for the good advices! I am planning to cover a wooden hull (strips) with glass fibre and epoxy. I have used Plastic Padding filler and then sanded. But how smooth should the hull be before covering? Of course no bumps or holes, but how even or uneven would be ok? Thanks.

Epoxy not going off.
by: Mike John

Good article. I think if you are building with wood, epoxy is the only way to go. A cheap set of electronic scales from the supermarket can help mixing accuracy. If it does not go off, a little more epoxy with the right hardener will make all of it go off including the underneath epoxy.

Mike John

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