7 Common Mistakes when Working with Fiberglass and Resin

Fiberglass and resin are two popular materials used by boat builders, car restorers and other fiberglass enthusiasts. It's often used in hobby and DIY projects because it's relatively inexpensive and easy to work with. However, there are a few common mistakes that people make when working with fiberglass and resin. In this blog post, we'll talk about 7 common mistakes and how to avoid them.

XZH contains other products and information you need, so please check it out.

1. Mixing too Much Resin at one Time

It’s important not to mix too much resin at once. You want to make sure you take the time to apply the resin in small batches. If you rush and make one large batch, the resin may get extremely hot and may even start smoking.  This is called exo-therming. It can kick off in the mixing container before you have a chance to use it. Be aware that the room temperature will also influence how fast resin will begin to harden.

Allow enough time for your project. The time needed will depend on how big the project is and could vary from several hours to several days. This is because you need to allow for curing time between layers or pours. Follow the manufacturer’s pouring/mixing instructions for your product so you have a successful project.

2. Not Using Enough Resin

Using too little resin with fiberglass can also be a problem. If you don’t use enough resin, the fiberglass may not stick to the surface properly. It can also lead to fiberglass fibers fraying and your final product could be brittle and weak.

Make sure to prep your surface by doing a coat or two of resin on the substrate first. This is especially important if you are working with a surface such as wood or balsa core. These surfaces will absorb a lot of resin. If you coat the surface first and let it cure, your laminate will not lose much, if any, resin and it will create a better bond.

On the flip side, you don’t want to use too much resin. You can use a roller or spreader to get rid of any excess resin you notice when laminating.

3. Adding too Much or too Little Hardener

If you add too much hardener to your resin, it will cure too quickly and will be very difficult to work with. On the other hand, if you don't add enough hardener, the curing process will take much longer than usual if it cures at all.

The room temperature will be a factor in the curing process as well. You may need to adjust the amount of hardener if you are working with polyester or vinyl ester resin (slightly more for cold weather and slightly less for hot weather). **It is important to note that you cannot vary the hardener amount when working with epoxy resin. Epoxy requires exact amounts of resin with hardener or else it will not cure correctly. **

If you are not sure how much hardener to add to your polyester or vinyl ester resin, you can check out our MEKP catalyst chart here- https://fiberglasswarehouse.com/pages/mekp-catalyst-quantity-chart . If you are working with epoxy, it is typically measured as a 1:1, 2:1, 4:1 or 5:1 ratio by volume or weight depending on manufacturer. Make sure to follow the instructions on the bottle for exact measuring instructions.

Adding Chopped Strand Mat Unevenly

When applying chopped strand mat, make sure that you apply it evenly across the surface. If you apply the fibers too thick in some areas and not enough in others, you may end up with weak spots or fiberglass-heavy spots. When you add the polyester or vinyl ester resin to the mat, the styrene binder will dissolve allowing you to move the fibers around. You can use a spreader or roller to not only work the resin into the mat but also to make sure you have a nice even amount of fiberglass and resin over the entire work area.

**Epoxy resin is not compatible with chopped strand mat.

Not Mixing Resin Thoroughly

It's important to mix the resin thoroughly in order to avoid any soft uncured spots in your project. Use a tongue depressor or stir stick to mix. Make sure to scrape down the sides of the mixing cup as you stir it so the resin and hardener are mixed evenly. It is especially important to mix well when working with epoxy resin.

Using the Wrong Resin

When working with chopped strand mat (CSM), you need to use a polyester or vinyl ester resin rather than an epoxy resin. Epoxy resins will not bond properly with most chopped strand mat and will result in a weaker final product. This is because chopped strand mat is made up of short fiberglass strands held together by a styrene binder. This binder needs the styrene in polyester or vinyl ester resins to dissolve in order for the mat to become pliable.

When doing a repair, you should find out what resin was originally used. If the part was made with polyester resin, you can use epoxy resin over top. Epoxy resin will adhere to polyester resin surfaces. The opposite is not true. If a part was made with epoxy resin and you use polyester resin, the polyester will not adhere well. If you are making a part and are planning to gel coat it, make sure you use polyester because gel coats are typically colored polyester resins. This will ensure your gel coat bonds well to the part.

Not Working in a Well-ventilated Area

Resin fumes can be harmful if inhaled for extended periods of time without proper ventilation. Always work in a well-ventilated area such as outdoors or in a garage with the door open when using fiberglass and resin so that harmful fumes are not concentrated in one area. You could also open a window to allow fresh air to circulate.

It is also important to wear a respirator mask and use the proper safety clothing to avoid getting resin in your eyes, nose or on your skin. If you skip this part and do not wear the proper safety equipment, you could end up inhaling toxic fumes, burning your skin or ruining clothing. Use a respirator, safety goggles and gloves. You could also get a spray sock and Tyvek coveralls if you want to further protect yourself.

Conclusion

When working with fiberglass and resin, it is important to take care and avoid making common mistakes. By following these tips, you can ensure that your project turns out safely and successfully.

For more information on fiberglass and resins, check out our Ultimate Fiberglass and Resin Guidebook for Beginners .

Hi,
I’m trying to use some F/G mat with West System epoxy. The mat does not want to hold to the shape of the panel I’m trying to patch at all. The West System Tech Support people tell me it should work, I’m wondering if anyone’s had any success with it. The mat was kindly given to me by a friend so I have no idea of it’s age. Thanks.

Best,
Jonny

Well the section on bonding in bobbins was most illuminating. No wonder it didn’t work, I was doing it all wrong.

Want more information on fiberglass mat for sale? Feel free to contact us.

The manual references 1 1/2 oz laminate. I am using 6 oz cloth but it is pretty thin stuff so I’m inferring that they are referring to mat. Since I am working with epoxy I will use some nice thick 18oz cloth. That should be beefy enough.

Anyway, thanks again Alan for your helpful post.

Matte contains a sizing that binds the loose fibers together. It gives the matte a stiff feeling, like a startched shirt.

Polyester resin contains solvents that quickly soften the sizing, causing the matte to go limp and easily conform to contours.

Epoxy resin contains no such solvents, and the matte remains stiff for a longer time. But given time, soaking in the epoxy, the sizing will soften and the matte will become pliable. Just don’t get in a hurry and mix a ‘hot’ batch of epoxy… you need to allow a little time. Be patient, wet it out, let it sit for a while, then go back and work it.

Or… mix a little acetone or ketone into the epoxy. The solvents will attack the sizing, then being volatile, will evaporate. They may cause the epoxy to take longer to cure, but it will still cure. Try a test piece before committing a batch to your project.

Regards,
Tim Engel

Hi Tim,

I knew what the problem was I just didn’t know what the cure was. Thanks for this, I will give it a test shot tonight.

A very kind gentleman in Atlanta is going to send me a 10 sq ft “sample” of his company’s 1/4" needled mat. Although it is thick there is no sizing in it to mess things up. I’m hoping that it will not be too thick once I’ve added the epoxy. That however is for later, tonight I’ll test your method.

Hi Alan,
I think that what I have is 6oz cloth which is about twice as thick as a dress shirt. I will experiment with mat this a.m. My son is bringing some 18oz cloth from the Big City tomorrow which I shall play with.

I’m keen on cloth as it gives an inherently stronger repair as there is less resin in it.

Thanks for your help and support, I’m having fun learning.

Imho you are better with Chopped Strand Mat/roving/shredded as used by Lotus to make the cars.
With the Chopped Strand Mat the first piece is the smallest then the following slightly bigger and so on.
This way the stress is spread over a larger area. This stops stress cracks caused by stress being in one concentrated line.
Have a close look at the other Bobbins on the car and how they were laminated by Lotus.
Or buy the book written by Miles Wilkins about grp.
The only place i know where you always get stress cracks on a +2 are next to the Headlamp in the square corners (body).
Imho this was an oversight by Lotus and they should have made corners with a big radius or laminated a rope underneath to spread the stress.
When i need grp materials i always go to a boat yard or chandlery for top quality products.
Well anyway good luck and don’t forget to wear rubber gloves to stop hairy fingers

Alan

I think that you’re correct about the CSM.

It is worth noting that one of CSM big advantages in a production situation is that it can be sprayed in using a fancy gun whereas cloth requires hand work. So I think that CSM was used by Lotus’ suppliers to keep costs down. I don’t think that this means that you have to use CSM to repair them. I’ll experiment with CSM in about an hour or so . . . and . . . . I’m one of those people who has to learn by experience . . . . so that’s what I’m going to do. The other catch is that I’m using Epoxy which makes CSM a bit of a pain to work with.

Thanks for your straight talk, I’ll report back when I’m done.

“Good judgement comes from experience . . . experience comes from . . . bad judgement” - Anon

Woven Roving (ie, ‘cloth’) is stronger, but the weave pattern can imprint through to the surface and be visible through the paint. If you start with a bare mold to make a new part, laying in a few layers of matte first, then a layer of woven roving 3+ layers deep can imprint through to the surface. Yes, 'tis true.

For structural composite panels, you’re right, woven roving can produce a stronger part “IF” you’re smart about how to orient the fibers. If you’re laying up a composite fuselage for your homebuilt aircraft, use woven roving. But for cosmetic panels (car body panels), matte is better since it will not imprint any pattern thru to the surface.

If you’re putting another layer on the inside of an old Lotus panel, then it’s probably not a big deal what you use, other tan matte will conform more easily around abrupt surface contours… like around a bobbin (then wrap it with some linear roving.

However, if you’re making any external repairs, like fixing stress cracks, then avoid woven roving and veil (very fine, 0.5 to .75 oz woven roving). Instead, use matte or tissue (0.5 - 0.75 oz matte).

Off Topic, but…
A friend of mine used to own a Countach. The body was mostly metal, with a few graphite pieces mixed in… like the front bonnet. Over time, the weave would imprint through, and Countach fans consider the weave pattern a badge of honor… proof of the car’s originality. One time he had some body work done to the front… nothing that involved the bonnet. The body shop took it upon themselves to do him a favor while they were working on it, and ‘fix’ that crude bonnet. It came back looking like a mile deep black mirror. Perfection. Their hearts were in the right place, but they really ticked him off… he pissed & moaned that it would take years for any hint of that weave pattern to come back.

Regards,
Tim Engel

Well I managed to make it work by using Tim’s advice. I put in a tablespoonful of Ace Tone (great name for an amp or a band) in with the epoxy mix and let the pieces of mat soak in epoxy for 15 minutes. The mat wouldn’t follow the contours very well but I kept at it until it started to set up and then I was able to persuade it to stay down. I had to come back every 5 mins and re-persuade it but eventually it stayed down.

I think part of the issue here was that I was putting the mat over an outside corner (bobbin in the rear suspension tower) so it had a strong tendency to fold up and away from the original F/G. Next I’m doing a floor bobbin so that will be inside corners. I’m hoping that’s easier.

Thanks much for all your help and suggestions.

For more information, please visit fiberglass mosquito net.