How to Fiberglass Like a Pro
Fiber bonded with plastic resin has a long long history. As with many manufacturing techniques, fiber reinforced plastic came into wide use in the military. Part of the reason for this is the ease with which objects can be created by combining these two materials. It is also quite strong when compared with an equal weight of steel. Fiber reinforced plastics can be as much as six times stronger than steel at an equal weight.
But the real attraction here is object made with this material require no high temperatures, a modest collection of tools, and best of all, very little prior experience. In fact, if you've built a sandwich and painted with a brush, you know most of what you need to know about fiber-glassing.
What is Fiberglass
Fiberglass, as a term, is really a misnomer. Glass fiber is one of the many materials that can be used along with resin (plastic) to create a strong and light composite structure. Those materials include glass-fiber (of course), cloth (cotton or man-made), carbon-fiber, non-woven glass or carbon mat, coir (a coconut fiber), modal (beech tree fiber), bamboo, and even hemp fiber.
In truth what most people call fiberglass is properly referred to as Fiber Reinforced Plastic (FRP). It is a composite material, much like Carbon Fiber Reinforced Plastic (CFRP) so popular today, but FRP predates carbon fiber by at least seventy years.
One of the earliest uses of FRP was by the United States military in making helmet liners. (see picture at lower right for a World War II liner)
After the war returning GIs began experimenting with the material in the form of car bodies and boat hulls. Sailboats were some of the first items built with this material and a few years later powered watercraft. Because so little was known about the strength of FRP in those years most of those items were "overbuilt." Boat hulls and car bodies made in the 50s and 60s are still around today almost indestructible in their strength.
Ideal Hobby Material
In many ways it is an ideal hobbyist material. With a bit of patience it is fairly easy to use with most of your effort going into preparation if not actual creation of the item. No special tools are required. In fact I recommend using old food containers to mix the resin in and purchasing or using cheap bristle paintbrushes, plastic trowels, and/or wooden dowel for applying the resin.
Not Much Dexterity Required
There isn't that much dexterity required. If you can lay a sheet of fabric out and remove all of the wrinkles you are halfway there. If you can lay on a thick paint-like liquid without leaving behind any bubbles you are one hundred percent of the way there. It is really pretty easy.
The only other "ingredient" is patience while waiting for the resin to harden.
FRP can be used to make auto body repairs, boat repairs, or even to create new objects that have not existed before. It is also possible to use FRP to create molds for other FRP objects. Some examples of past FRP use include Eames bucket chairs, lamp-shades, decorative partitions, green house walls, and door panels just to name a few.
If you've ever had or sat in an Eames bucket-style chair you know how strong and light they are.
Fiber reinforced plastic, by some estimates, is six times stronger than steel for the same weight in materials. It is strong enough, in fact, for use in fiberglass swimming pools (water is quite heavy) and even fiberglass buildings.
A Note on Possible Uses for FRP
After a few questions regarding specifics about using fiber reinforced plastic techniques I thought it best to add this note.
A Plethora of Possible Uses
Though this article is roughly 3,000 words long it by no means covers every type of fiberglass application you can use. It can't. The document would have to be at least one hundred times longer to cover every possible use and technique for a variety of situations.
When making repairs to existing fiber-glass objects you only need to pre-wet the area with resin, add the fiber, and then re-wet the new fiber with resin. For esthetics this is usually best done from the inside of the object.
Strengthening Existing Objects
If you are laying a new fiber cloth coating to a boat hull, or say a wooden deck, you don't want to pre-wet with resin, but you do want to let the cloth sit on the surface for a number of hours before adding the resin. This is so the cloth conforms to the object. Only after all of the wrinkles are out of the cloth and it conforms to the object being strengthened then you add any resin.
Even then you don't use a lot of resin. Only a small amount of resin (about a 1 cup per ten square feet) is required depending on the size of the object. Then you want to work the resin into the cloth as thinly as possible. Even then the amount of resin used is variable due to the different absorption rates of various types of cloth/mat.
Work Inside Out
If you are repairing a hull breach (a hole below the water line of a boat) you will want to repair from the inside of the hull, not the outside. To effect a good solid hull breach repair you'll have to make the inner hole three times the size of the outside breach to make an effective repair. The hole should be cone shaped too so that the outside of the breach is no larger than when you started. This is done to make the strongest possible repair to a hole.
If you are making a new object you want to work from the outside in; just the opposite of above. This does not even mention the necessity of a mold, what the mold is made of, and what you need to do to get the new object to release from the mold. However, you can make molds from wood, plasticine, or even existing objects. The mold will have to be perfect, smooth, waxed (wax is a releasing agent) and very clean.
This is not even the "tip of the iceberg" as to techniques; it all depends on what you are trying to do.
So, at best, this is an overview of how to use fiber reinforced plastic.
Fiberglass Helmet Liner
Fiberglass is the actual cloth or mat made up of glass fibers. As noted above the fiber DOES NOT have to be glass. However this section will cover glass fiber specifically.
The list below contains common designations for this material.
- Woven Roving
- Glass cloth (AKA bi-axial)
- Woven fabric (AKA bi-axial)
- Glass mat, this material has no weaving or set orientation
- Knitted fabric has the typical cloth pattern with additional runs of fiber at forty five degree angles.
The fiber in FRP does not need to be glass fiber either; it can be cotton, linen, bamboo, coconut fiber, beech tree fiber, or even hemp. Carbon fiber is "burnt" Kevlar embedded in resin. So in actuality carbon-fiber objects are just another form of FRP.
Preparation is Everything!
Well, almost everything. You'll need the right weather conditions or temperature and humidity, the right tools, and the right surface preparation to get a quality job. Likely, you'll also need to know how long it takes for a piece to cure, when you can add to it, and how to protect it once it's finished.
Weather is very important. Ambient temperature should be between sixty (60) and eighty-five degrees (85) Fahrenheit or between sixteen (16) and thirty (30) degrees Celsius.Temperatures below sixty (16) will mean extremely long cure times or no cure at all. In fact if ambient temperatures are at or near freezing the resins won't cure at all.
Relative humidity should be below sixty percent (60%) though epoxy is not as susceptible to humidity as poly-resin. You do not want to layup an FRP job in the rain. Trapped moisture will not stop the curing process or even slow it down, but water bubbles in your layup can cause the layers to come apart months or years later. This in turn will compromise the strength of your creation so avoid if possible.
Fiberlgass MaterialsClick thumbnail to view full-size
Note: I use yogurt cups, pudding cups, cottage cheese and cream cheese containers. I measure a set amount of water into one of the containers and use a permanent marker to draw a fill line on the outside of the container. This way I have a disposable mixer/measuring cup for measuring the resin components.
If you are using a polyester resin only the resin needs to be measured; the catalyst is a few drops of liquid. If using epoxy you must measure both the resin and catalyst so two measuring containers may be required.
ToolsClick thumbnail to view full-size
Tools & Materials
Basic Tools: You'll need sandpaper, acetone or denatured alcohol, tack cloth, distilled water, glass cloth, resin, mixing containers, and a paint brush or roller. Patience too.
Tools are probably the cheapest part of this. The cloth the next cheapest and the resin the most expensive of all the items required. I strongly recommend using Polyester resin for this first attempt as it is cheaper and more forgiving.
Safety Wear: Since the resins are hard on your skin you'll want to gloves and eye protection. A long sleeved shirt, shoes, and long pants (not shorts) should be fine.
Cutting: I recommend an old pair of scissors. The cloth is much harder to cut with a knife though it can be done.
Mixing Containers: Of course you'll want an accurate measurement of your liquid components, but you can get an inexpensive measuring cup at a paint or hardware store.Save your money on mixing containers. Since you don't want to start out with a huge job you can do this with old margarine tubs, or other used food containers. Just be sure they are clean and dry. This way you won't feel bad throwing them out afterward either.
Spreading/Leveling Tools: Use old cheap paintbrushes or the cheap synthetic paintbrushes. The brush does not have to be the same high quality you'd use on a paint job because all you'll be doing is spreading the resin around. Do not use a foam brush; it will melt with polyester resins.
You'll also want 1/2" thick wooden dowel material to serve as a roller. You'll use this to make sure all the air bubbles are out of your work. You could buy a specialized fiberglass roller, but they cost more than dowel, are harder to clean. A specialized resin rollers is only worth the cost if you are planning on doing a lot of FRP work. Wooden dowel, on the other hand, can be cut to the right width (about six inches) and thrown away without much worry.
Sandpaper: You'll want a 60 grit and a 120 grit at least. If you are doing finish work (work that will be painted) use higher grits in the 300 and 400 range.
Solvents: Like acetone and/or denatured alcohol are used to degrease and clean the areas you are going to lay FRP on top of. Oil and grease will prevent the resin from sticking (this is a hint about release agents) so you don't want any residue, not even the oil from your hands, on the area to be fiber-glassed. Acetone will remove most types of paint too so start with alcohol until you gain some experience.
Final Cleaning: Your final cleaning of the FRP surface will require a tack rag and distilled water and lint free cloth. This is probably not really necessary, but I always go this extra step and have never been sorry.
This is basically what you'll need. Oh, and the reinforcing fabric and resin of course.
The "F" in FRP
There is a wide range of cloth that can be used for the fiber component. The most commonly thought of for FRP is fiber-glass, but you don't really have to stick with that for project that don't require a lot of strength.
Cotton, hemp, wool, kevlar, carbon-fiber, in fact any natural or human-made material that will bind with the resin will work well as the fiber part of the FRP. It doesn't even have to be woven cloth; it can be a "felted" cloth with no weaving.
If the object you are creating or reinforcing needs to be strong then the cloth component should be woven. Woven cloth is strong in tensile (pulling apart) strength and with the resin embedded into it is also strong in compressive (pushed together) strength.
If the object you are creating only needs to be strong enough to support it's own weight plus some and is primarily decorative then a non-woven (felted) cloth works well too.
Types of Cloth Used in Fiber Reinforced Plastic
Lamp-shade , room partition
Upper Body (boat or car)
polyester or epoxy
Tublar frames (bicycle)
Boat Hull or Car Body
Preparing and Glassing your Surface (Repair)
- Figure out how much resin you are going to use (assuming polyester). If you are fiber-glassing a one foot square area figure on one cup of resin and however many drops of hardener recommended by the manufacturer. Do not prepare the resin yet. The resin will start to cure as soon as it is mixed so you want to do this just before you apply it.
Assemble the tools you'll need. Cloth, resin, gloves, glasses, measuring containers, dowel or roller, sandpaper, alcohol (denatured), water, clean lint-free cloth, and paintbrushes.
- Prepare the surface to be fiber-glassed. You want to use a rough sandpaper and remove any paint, varnish, oil, and/or dirt. It's best if you sand right down to the original surface. You can use a power sander if you wish. You do not even have to get the surface particularly flat.
- This is the point at which you want the gloves and safety glasses. Clean the surface. You want to vacuum it or use denatured alcohol on a lint free rag to get any of the sanding debris off. Do this two or three times to make sure that the surface is perfectly clean.
- Clean the surface a final time with distilled water and a lint free cloth. Let air dry or blow dry with compressed air. Use the tack rag last to make sure every contaminant is gone.
- Cut your fiber cloth to the size of the repair. The easiest way to do this is to cut the cloth, hold it to the repair area, and trim some as needed. Now that you have the right dimension set the cloth aside on a clean work surface.
- Prepare the resin. Following the manufacturers instructions pour the resin into the measuring cup to the recommended amount (see above) and carefully add the hardener drops. Stir this mixture thoroughly and slowly. Avoid churning air into the resin. You'll notice it's about the consistency of pancake syrup. If you are using epoxy then you are looking at a half and half or possibly two (2) parts resin to one (1) part catalyst. Polyester resins use MEKP (a few drops) as a catalyst.
- Apply a layer of resin S.L.O.W.L.Y. Because of the viscosity it will tend to run off of your paintbrush in long strands and get everywhere. Working with it slowly will ensure it goes where you want it to go.
- Apply the cloth layer (You should have your gloves on remember?) and lightly press it into the resin with your fingertips. It should stick readily. Now, add a little more resin on top of the cloth. As you do this use the paintbrush to spread the resin evenly; you want it to penetrate the cloth and leave a thin layer of resin on top. if you see any air bubbles brush them out. You can also use the doweling if necessary to press the resin/glass sandwich together and remove any additional air.
Air bubbles are bad; they weaken the composite.
- Wait! Curing will take from two to four hours depending on the resin. Check the manufacturers label.
Ambient air temperature is important. Most resins need to be twenty or more degrees above freezing to set. Check the resin label for ideal temperatures and humidity levels.
If you are planning to add another layer watch this cure time closely. The resin will cure to a hard, yet sticky state after the prescribed (two to four hours) amount of time.
When the resin is hard and sticky is the ideal time to add another layer of resin and glass without any preparation. Mix another batch of resin, apply it, the fiber (cloth or mat) and more resin. Roll out the composite with dowel of brush out bubbles and wait the prescribed curing time. You can do this many times as long as you add the next layer before the resin cures to a non-tacky surface.
If you let the resin cure for a full twenty-four hours you will have to sand it and clean it as outlined in the third and fourth steps above.
If it's getting late in the day and you have leftover pre-mixed resin it can be put in a covered container and stored overnight in your freezer. It will not cure at or near freezing. The next day pull it out of the freezer and as soon as it thaws use it.
Using the method described above successive layers of glass and resin can be layered, ending with glass mat for a paint-able surface. I strongly recommend using a primer then finish paint for a professional look. Paints should be polyurethane or two-part epoxy. The two part epoxy paint is tougher, but harder to come by (some states now ban it's use by all but certified professionals). Epoxy paint is also better for "immersion service" e.g. use below water.
Note that the pigment (and often additives) in the paint prevent epoxy based paints from reacting to ultra-violet or sunlight.
New Surface vs Repair
All of the steps above assume you are making a repair.
If, on the other hand you are adding a new fiber reinforced coating, to say a surfboard, boat bottom, or other undamaged object you do not want to pre-wet.
You do want the fiber cloth to sit on the object for a few hours, overnight is best, so its shape conforms to the object. You will then apply the resin once you are sure the cloth conforms to the shape of the object. If you've got wrinkles smooth them down with your clean dry or gloved hands.
Because it is likely just you doing this, and you don't have a team, limit your work area to three or four square feet. There is no reason why you can bond fiber fabric to your surface in steps.
Add the small amount of resin, half a cup or so, and work it into the cloth over and over until the brush or putty knife starts to drag. You want to do this to keep the cloth down against the surface until bonding starts to take place.
Avoid working from the center out. This just complicates things. Work from one edge down. Your object is to work the resin completely into the cloth and stretch that resin as far as you can. Resin, once cured, is really strong so a thin layer is not only acceptable it is desired.
If you are adding another layer, wait the prescribed cure time, before adding the next layer. If you are using cloth, try putting the next layer down so that the warp and woof (the threads in the fabric) are at a 45° angle to the previous layer. As before work in sections and work the resin as thin as possible.
Do you have a snag in the cloth? With your forefinger, starting from the outside of the snag and working in, draw an every tightening circle with your finger working in toward the center. You can do this before adding resin or after (because sometimes snags occur after adding resin). You may need to "circle" the snag a number of times, but this method is effective.
The video below is an excellent example of fiberglass layup from start to finish. It's a time-lapse presentation so you may want to hit the pause button from time to time to grasp the finer points. There is also commentary embedded in the video to explain what's going on.
I find it particularly useful since the videographer also used releasing agent at the beginning. You see why that agent was used at the end as well as what the part created is used for.
And what is the releasing agent? Ordinary car wax without any additives.
Timelapse of Fiberglass Layup
Notes on the Above Video
- He puts wax on his mold as a release agent. This makes taking the cured part out of the mold much easier.
- Notice he puts masking tape around the edge of the mold. This give him something to grab when it's time to separate the part from the mold.
- The first thing applied is called GelCoat. It is colored resin without the fiber reinforcement.Just prior to spraying it on he adds the catalyst and then stirs the mixture. You'll notice it's pretty thick; this is fine and should be thick. Once cured the gelcoat has a very smooth finish against the mold and a rougher finish on the side fiber will be added to.
- He applies seven layers of resin and five of fiberglass. You'll notice the first three layers of fiberglass are held in place with a blue resin. This is polyester resin.
- The last two layers are held in place with a yellowish resin. This is epoxy resin.
- With seven layers of resin and five of fiberglass these items are very very strong.
- At some point he uses a bandsaw and drill to cut the pieces out and create holes. These steps are not shown in the video though the final product is.
- If you haven't guessed by now these items go just over the headlights and are held in place by two screws under the hood. They give the "nose" of the car, a Mistubishi if I'm not mistaken, a slightly different look.
The author has received no money, discounts, or freebies for any products that may be mentioned in this article.
A Note About This Hub
Again, this hub is geared toward reader who is considering fiber-glass as a hobby. Because I've been getting comments lately toward more professional, salable results I want to remind the reader what this hub was written for. Many, if not all, of these additions involve techniques and materials geared toward high-volume, fiber-glass product production.That is not what this hub is about. This hub was written for the hobbyist with frugality and reasonable costs in mind. Since this is the the idea behind this hub, I will not be adding much more to it, especially if that "more" involves materials and techniques geared toward high production, high volume, or high cost to the reader.
Additionally, there is no way an article of 3,000 words can cover every technique necessary for working with fiber reinforced plastic. Repair techniques are necessarily different than those for creating a molded object, and are different yet again for creating something like a surfboard.
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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.