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Fiberglass Repair and Bonding

There’s a couple threads about repairing ABS enclosure crakcs, but not much dedicated to fiberglass. I’d like to learn from the fiberglass masters on good technique for getting creative with fiberglass.

A few starter questions:

  • Can you glob on 2-part epoxy to a fiberglass enclosure and get a good bond?
  • Is polyester resin better than epoxy for cloth layup?
  • Is there a large difference in quality resins (West System vs HomeDepot special)?
  • For repairs/modifications, what’s your preferred type of fiberglass cloth, mat, tape, weight, weave, etc

The selfish reason for this thread is that I plan to cut a fiberglass enclosure in half, and I’d like to know the best way to glue it back together after I remove some length. Any input appreciated.


To contribute, I’ll add quotes from my research:

Polyester vs Epoxy:

Polyester resin is excellent for fiberglass lay-up — building a boat, for example. Polyester laminating resin doesn’t fully cure while exposed to air, so when it is used for lay-up, every application joins to the previous one on a molecular level. You end up with what is essentially a single substance encapsulating multiple layers of glass fabric.

However, when you are doing a repair, you need for the resin to also function as an adhesive — gluing the patch to the surrounding surface. Polyester is an adequate adhesive but not as good as epoxy. As a general rule, the tensile strength of a polyester bond will be around 20 percent weaker than the same bond made with epoxy. That makes epoxy resin usually the best choice for fiberglass repair work. (Polyester or Epoxy Resin? | BoatUS)

SilverTip Epoxy | Laminating Epoxy Resin – System Three Resins

If you are repairing a polyester product, or non-critical part you should use a polyester resin. If your application is gluing, load bearing or repairing an epoxy part, then an epoxy resin should be considered. In any case, the surface that you are repairing will need to be scored before applying the resin. It should also be clean and free from impurities. (Polyester or Epoxy? How to choose your Resin)

Epoxy adheres to wood much better than polyester does. To cover wood, a laminate using epoxy resin and 10 oz, cloth will yield a much better job than 10 oz. cloth and polyester resin. The epoxy/cloth laminate is comparable to using polyester resin with 3/4 oz. mat and 10 oz. cloth however the epoxy laminate requires much less labor to fair.
A polyester laminate job is somewhat less expensive in material costs than an epoxy laminate project of the same size. (WHICH RESIN TO USE?? EPOXY vs. POLYESTER vs. VINYLESTER – LBI Fiberglass Products)

Reinforcement (fibers):

The main reinforcement materials used are Glass mat (fibreglass), Aramid (often called Kevlar) and Carbon fibre. Fibreglass can come in the form of chopped strand mat, surface tissue and woven roving fabric. Carbon fibre and Kevlar come in fabric with different woven patterns.

Generally, you can associate any reinforcement material with polyester. Epoxy tends to be used with either Aramid or Carbon, but can be used with fibreglass. It is important to keep in mind that if you are using fibreglass mat with epoxy resin, it has to be a powder bound mat. Emulsion bound mat contains styrene only dissolved by polyester resin, so it shouldn’t be used with epoxy.

Chopped mat should be used for general repairs, while woven glass, carbon fibre and Kevlar should be used when extra strength is required. When using woven roving, a layer of chopped strand mat should be used as the final layer to prevent peeling.

If you are using fibreglass mat with epoxy, make sure it is powder bound, there is no constraint using polyester or epoxy for other reinforcement materials. Always ensure that the correct amount of resin is applied to the mat. (Polyester or Epoxy? How to choose your Resin)

Fiberglass mat should not be used with epoxy because the binding material in the mat will NOT dissolve in epoxy resin. Either mat, cloth or woven roving may be used with polyester or vinyester resin. (WHICH RESIN TO USE?? EPOXY vs. POLYESTER vs. VINYLESTER – LBI Fiberglass Products)

The average fabricator can choose from three common reinforcements: fiberglass, carbon fiber, and Kevlar®. Fiberglass tends to be the all-purpose choice, while carbon fiber offers high stiffness and Kevlar®, high abrasion resistance. (

Once the family of fabric has been determined, select the weight and weave style which suits the demands of the job. The lighter the ounce-weight of the fabric, the easier it will be to drape over highly contoured surfaces. Lightweights also use less resin so the entire laminate remains lighter. As fabrics become heavier, they become less flexible. The medium weights retain enough flexibility to drape over most contours and they contribute significantly to the strength of the part (6-, 7.5-, and 10-ounce fabrics are our most popular). They are very economical and produce strong and lightweight parts used in automotive, marine, and industrial applications. Woven roving is the heaviest reinforcement and is typically used in boat building and mold construction.

Fiberglass MAT vs CLOTH:

The difference in fiberglass cloth and mat is in the way it is constructed and in its appearance.

  • Mat: The fibers are typically three to four inches in length and are randomly oriented. Chopped strand mat is not a very strong material because of the short fiber length. However, it is isotropic. This means that it is equally strong in all directions (mat and fillers are the only composite reinforcements exhibiting this trait). Mat is the least expensive reinforcement form and is thus the most widely used. It is suitable for molds and part production. The random orientation effectively hides fabric print through of gel coats and makes molds which are equally stiff in all directions. It should be noted that chopped strand mat is only compatible with polyester resin.

    Chopped Strand Mat: 1 1/2 ounce

  • Cloth: Woven fabrics are strong reinforcements because the fibers are bundled into yarns oriented in just two directions. The warp and fill yarns run at 0 and 90 degrees respectively. Thus, fabrics are anisotropic, or strong in only two directions. Fabrics need to be oriented so the fiber yarns run parallel to the expected loads. If extra strength is needed in a different direction, another ply must be added at an angle to the first. The most common angles are +/- 45 degrees.

    10 oz Fiberglass Fabric

Repair Tips:

Repairs Differ from the Original Part -Once a part is damaged, all repairs become secondary bonds attached to the original primary structure. This means all repairs are dependent upon physical bonding to the surface of the original primary structure (more on this later). For this reason, fiberglass repairs rely upon the adhesive quality of their resin for their strength—the strength of physical bond to the primary structure. Because of this, the resin used for the repair should be just as strong as the resin used to fabricate the part. In fact, resins with strong adhesive properties are sometimes used for repairs.

Increased Surface Area Increases Strength — Since fiberglass repairs depend upon surface adhesion (physical bonding) of the repair to the primary structure, increasing the surface area of the bond line will increase the strength and durability of the bond—and by extension the part or repair.
Typically, the method employed to increase the surface area is taper or scarf sanding. This type of sanding means the area next to the damage is sanded away gradually, generally resulting in approximately ½—¾ of an inch of area per ply of composite laminate.

Repairs Should Match your Original Part — While your composite repair is different than your original part, it is recommended that you duplicate the thickness, density, and ply orientation of the original laminate when making your repair. This will help to maintain the functionality of the part. More is not always better—in this case, if your repair is thicker than the original part, it will almost certainly be stiffer, regardless of the material in use. Introducing various strengths within a part can cause unintended stress points, eventually leading to material fatigue or failure. It is better to carefully replace every ply that has been removed in the damaged area with an identical material, placed in the same orientation when possible. This ply—for—ply replacement approach guarantees the repaired structure can withstand the same loads as the original, and that it will disperse loads as intended.(


To repair your enclosure I would cut both sides with a radial arm chop saw to get both perfectly straight 90°. After that I would mate them up and see where they dont touch and proceed to the belt sander to make it perfect. Once they are matched together tight very little epoxy will be needed to make the bond. While your sanding make sure to sand the inside down to a taper. The taper on each side will allow you to lay up several layers of cloth and keep it from building up to much. I would bond the two pieces together first , followed by an overnight cure, then do the inside the next day.


Taper sand both the inside and outside?

Edit: Ah, I think I understand. Cut > sand + taper > epoxy edges > cloth the inside

The other (lazy) option I’ve been considering is using the leftover pieces of enclosure (the section I cut out) and epoxying that right over top of the joint to reinforce rather than laying new cloth. Is that a terrible idea?

It won’ t fit. But if you had two strips of the same size you could cut them to meet in the middle. You could then epoxy the 2 part strip to one side to make a lip…then you would have an access panel for easy inspection but if wont be waterproof unless you get creative with thin neoprene . It would look better if the lip was on the inside but not sure about battery clearance

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True, it won’t fit directly over/under, but I could chop it into horizontal and vertical sections. That would cover 80% of the joint.

A lip is an interested idea. I considered that - one piece covers the battery, one piece covers the electronics. Height room is not an issue over the vesc / bms. Butyl tape would make a good waterproof seal. My big concern is the sacrifice to structural support - I think you’d lose some strength if you ever took a hit to the center/middle of that lip.

Feather both sides on opposite sides, making size to slightly overlap. Then, use epoxy, or even gorilla snot. You will have essentially no repair.

I need to reinforce my deck with fiberglass.

My deck is painted with spray paint. Is it a bad idea to put the fiberglass cloth directly onto the paint? Does it need to be bare wood?

I only thought about this AFTER painting it and if I can avoid removing all the paint, that would be a bonus.

It’s not gonna be as strong if you bond to the paint. You will be relying on the bond of the paint to the board for structural qualities. Not really ideal.


Yeah, that’s what I thought. Dang it. Better that I asked now before midway through the fiberglass layup!

I wonder what the best method for paint removal would be? Tried scraping, but that felt like it was gonna take forever. I’ve never used any paint strippers before… and I wouldn’t want to use something that could potentially affect the epoxy, so any advice is much appreciated.

Acetone. It should eat through the paint, and is the perfect product to use to prep the timber for epoxy anyway.

Typically i find it easier to sand off paint, but ymmv


Well hot diggity. A 2-for-1. AND it’s something I already have at home! Thanks!

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The other option is an orbital sander with rough grit sandpaper. That may actually be quicker than acetone. Depends on if you use finer grits to smooth the surface, but you could easily be done in 20 minutes.


I got this for an enclosure. Do I have to be outdoors to use it, How bad are the fumes?

It appears to be the kind that does NOT have extreme smelly issues, but I’m not a composites expert.

I think “traditional” means it’s not a polyester epoxy which means low smell. Not sure though

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cool, thx. currently using my home office for my work space.

Respirator. Then inside or outside isn’t an issue


cool…now what did I do with my organic vapor filters :thinking:

No bad smells, though I would still use it outside if possible.