About resins used to apply fiberglass cloth

All composite cloths are applied using some type of resin.  With the boom of composite construction have come a wide variety of resins specifically formulated to have the ideal properties.

• Safe to use.

• Compatibility with other adhesives and components it will contact as well as with the finish that will go over it.

• Adequate pot life.  The resin should stay workable for as long as it takes to apply the cloth to the largest part while allowing time to remove excess.

• Easily spread.

• Saturates the cloth quickly without having to work it too much.

• Reasonably short cure time.  Should be able to be sanded no more than 24 hours after application.  I like resins that are at least hard enough to be feathered about 6 hours after application so that I can apply adjacent pieces of cloth.

• Sands well without balling up or gumming in the sandpaper.  Good resins produce a fine powder when sanded.  Generally the harder a resin is when it cures, the better it sands.

• Non-brittle.  Harder resins tend to be more brittle so a balance must be struck to ensure the resin has some flexibility but also hard enough to sand well.

• Excellent adhesive properties so that the cloth stays down.

• Minimal soaking into the wood.  The deeper that the resin penetrates, the more filler that that will be needed.

Types of liquid finishes used to apply fiberglass cloth

There are several liquids that are appropriate for applying glass cloth.  I've used some, but not all of them.  After trying a few different things, I found something that I like so I have no desire to try anything else.  That's pretty much how all of model building goes.  There are no set rules, it's just finding the methods and techniques that you do best.

• Epoxy Glue

Epoxy resin of at least two types can be used.  Slow drying (30 minute minimum) epoxy glue is commonly used to apply glass cloth, but it is thick and does not penetrate the cloth well.  Regular epoxy glue is fine for things like glassing the center of a wing or similar reinforcements, but for large surfaces it just doesn't work as well as other types of resin.

• Epoxy Finishing Resin

Epoxy Finishing Resin (normally just called Finishing Resin or Laminating Resin) is designed specifically to apply composite cloths such as fiberglass and carbon fiber.  It has a thin consistency and a slow drying time.  It penetrates and saturates the cloth quickly and spreads easily.

I used 1 hour resin for the work done in this series of articles.  This is the first time I used resin that cures this slowly and I would not use it again.  1 hour epoxy cures so slowly that too much of it soaked into the wood.  This resulted in a lot more filler being necessary to fill the weave of the cloth.  More filler = more weight.  I have always used 30 minute resin in the past and have very good results with it.

The bottom line is that I recommend that you use 30 minute Finishing Resin for applying glass cloth.

• Fiberglass Resin

Fiberglass resin is polyester resin.  It is a two-part resin having a catalyst that I believe is peroxide based.  The more drops of the catalyst added to the resin, the faster it cures.

Note that polyester resin does not cure properly over epoxy.  Therefore, if you make things like fillets using epoxy and micro-balloons, you may run into problems if you attempt to use polyester resin over it.  Epoxy can be used over polyester, but there may be adhesion problems.  It's best to stick with one or the other and not try to use them both.

I do not use fiberglass resin because it smells horrible, causes bad rashes on my skin and makes me itch all over.  Polyester resin dries very hard and will sand better than epoxy resins sooner.  By that I mean epoxy resins take longer to achieve the same hardness.  I don't mind waiting, so I have not had a problem using epoxies, but some people complain that epoxies do not sand well which indicates to me they are trying to sand too soon.

In addition to the above methods that I have used, other people have claimed to use the following to apply glass cloth.  I have not used any of the following, so I can't speak from experience.

• Epoxy Paint (clear)

• Polyurethane Paint (water or solvent-based clear)

• Airplane Dope (clear)

• Cyanoacrylate (CA)

Some people will recommend that you use cyanoacrylate to apply glass cloth.  I think using CA is a really bad idea.  First, the fumes created are enough to knock out a stable full of horses.  Second, it is expensive and has a working time of zero seconds once it is applied.  Lastly, you have absolutely no control of where it goes.  It simple gets poured on and you hope for the best.

Resin and Fiberglass cloth are available from a variety of sources.  I purchase mine from Aerospace Composites.

Also see

• Adhesives for Model Building

Thinning epoxy resin (reducing its viscosity)

There are two ways to make epoxy glue less viscous.  Both methods can cause problems and I recommend that you don't use either of them.  I am providing this information because I am asked about it a lot.

• Use a Reducer

You should not use thinners because they alter the chemical curing process.

Some people add thinners (also called reducer) such as alcohol, acetone, lacquer thinner or MEK to the resin to make it easier to work with.

The result can be a rubbery or brittle finish.  Adding thinner to epoxy will cause it to shrink which it would not do otherwise.  Additionally, some thinners, particularly drug-store alcohol, contain water that can become trapped in the resin with unpredictable results.

• Use Heat

You should not warm epoxy because heat drastically speeds the curing process.

The other way to make epoxy glue less viscous is to warm it.  After the two parts are mixed, place the container into a shallow pan with warm water.

Alternatively, a hair dryer can be used to warm the epoxy after it is applied which will make it watery and easier to spread.  It will also soak in more.

Do not over heat epoxy!  The fumes are extremely dangerous.  Heat can also make the epoxy very brittle after it has cured which significantly weakens it.

You will have much less time to work with the epoxy due to the accelerated cure rate.  The resin may suddenly set up in the middle of the job at which point you're in trouble.

The only time I warm epoxy is when it is used as glue — not to apply glass cloth.

I strongly recommend that you don't use epoxy glue for applying a fiberglass finish.

West System Epoxy has an excellent article specifically addressing the benefits vs. consequences of thinning epoxy.

About squeegees

A squeegee is used to spread the resin and then to remove the excess.  Some people use old credit cards or a card from a pack of playing cards.  Others use commercial squeegees available at auto refinishing centers.

Personally I do not like any of the above choices.  I use a piece of medium 3/32" or 1/8" balsa.  I always have plenty of scraps of it and it has the perfect feel and flex to apply this finish.  It takes 30 seconds max to make a squeegee.

Typical Squeegees Top row — Commercial squeegees available at automotive refinishing centers. Middle row — Balsa squeegee and a laminated cardboard customer savings card. Bottom row — Playing card.  Newer decks make better squeegees.

How to Make a Balsa Squeegee Cut a piece of balsa to match the job.  For control surfaces, a piece 1-1/2" to 2" wide is about right.  For a fuselage or wing, a piece about 3" wide is good.  Make a couple because if you get over-zealous you might crack it. Sand one edge straight.  Round off the corner at each end.  The image at the left is kind of extreme - the edge does not need to be rounded so much. Knock the sharp edge off each side of the long edge.  A quick swipe with fine paper is all you need.  Don't round it over.  You now have a good squeegee.

No matter what you use for a squeegee, make sure the edge is smooth and free of nicks or other aberrations that will cause the cloth to fray or prevent even spreading of the resin.

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Copyright © 2003 Paul K. Johnson