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  • July 11, 2024 13 min read

    The simplest way to determine which resin to use for a project is deciding the application of the resin, and the strength and desired lifespan of a repair or build. Certain resins are ideal for certain constructions and can have massive benefits like weight savings, UV resistance, heat resistance, solvent and chemical resistance, flexibility or rigidity. Certain automotive fiberglass resins and bondo are not intended to be used in marine under the waterline, since they are automotive fillers, on the marine side we do offer polyester resins with additives that are compatible with winterization and underwater applications. Cost is always a factor as well, but the easiest way to resolve the cost is an analysis of the cost per square foot when using different resins versus only comparing gallon to gallon pricing. Certain resins also have limitations in their lay ups, compatible materials and resistance to temperature, etc., so one must have a basic understanding of how resins are used in different situations for the builder, manufacturer or repair situations.

    To begin, we commonly come across builders, especially ones that have done automotive bondo and small automotive repairs in the past, are using general purpose polyester resin for structural repairs and builds in boats, trailers, RVs, hot tubs, fiberglass decks. General purpose resin, often referred to by clients as fiberglass resin, is a low tensile strength resin for general applications, like campers and premanufactured goods. This resin is inexpensive and fast curing, and therefore, it is common in many industries. Manufacturers are using it as a backing for casting concrete or mold builds to increase the speed of production. The result is many trailers, campers, RV, motorcycle parts, car parts and boats that are easily damaged, cracked and UV damaged due to the limits of polyester resin. For small repairs this product can be used with enough fiberglass and plywood to reinforce trailers and boats, however, the layers and volume of fiberglass must be applied in enough layers and thickness to have strength. This always requires a certain volume of resin and cost that builders may not have anticipated.

    Gel Coat repairs on polyester resin are also common, where the gel coat crazes (spider cracking and delamination) due to UV damage, winterization, impact or water penetrating the Gel coat. Gel coat is Isophthalic Polyester Resin with pigment, which must be maintained with waxes and other products to stop it from having UV damage that leads to cracking. Gel coat is much like an egg shell coating on a fiberglass lay up, it is quite thin and prone to cracking if impacted, scratched or winterized outdoors. Gel coats on boats generally crack on the decks and hulls. In the hot tub industry, we commonly see the inner surface that cracks due to low water temperature or freezing outdoor temperature. For RV and trailers, its windows, doors and rock chips that cause crazing. In canoes and kayaks its almost always impacts or winterization that cause this crazing. What occurs is the resin is very rigid but brittle, and when impacted it will delaminate from the fiberglass (polyester resin) behind it and the fiberglass behind will also have micro cracks or large cracks. We also see a lot of gel coat wear when boats and sea doo’s are beached, this repetitive rubbing on the gel coat will make it thin which will have to be replace with new gel coat. This will have to be repaired structurally before it can be gel coated.

    There are two ways to apply gel coat but both require cutting and polishing for the desired semi gloss finish. There is build manuals and tutorials on our website for how to apply gel coat. In brief, the gel coat is catalyzed and applied with a brush. Following this the surface is wet sanded, cut and polished. For spray application, catalyst, air dry and styrene are added to thin and cure the gel coat. This is also cut and polished for the gloss finish.

    Polyester resin is compatible with chopped strand mat and most woven fiberglass cloths, but multiple layers of polyester resin with fiberglass should be applied for ideal strength. Our recommendation when applying new fiberglass to old polyester resin is a layer that is at least 3/16” thick. This would be two or three layers of mat with resin to be equivalent to one layer of woven cloth and epoxy. The volume required for the resin will be much higher than expected at approximately 12.8 square feet per gallon at 1/8-inch-thick application. At 1/32 inch thick the saturated area would be approximately 51.3 square feet per gallon. This does not account for resin application to 1.5 ounce chopped strand mat or porous plywoods, etc. There are other fiberglass cloths like e-glass and s-glass that offer the woven strength of the cloth and the strength of the resin, but they are more expensive than chopped strand mat (CSM). There are structural additives and powders that can be added to polyester resin to make structural putties and pastes, this is not Bondo. Bondo is a cosmetic filler that is not suitable for marine environments because it is very soft material that is prone to cracking when under water pressure or cold temperature cracking. According to the Bondo Body Filler technical data sheet, the filler has a tensile strength of approximately 1,000 psi, which makes it very soft when compared to marine grade polyester resin Polystar at 9,400 psi. Bondo can be used in above the waterline applications and RV, automotive applications, but we do not recommend large body fills or crack fills with bondo under the waterline on a canoe, boat or any watercraft.

    Polyester can be applied several different ways but the most common are hand application with an aluminium ribbed roller or chip brushes. Polyester resin can be used in mold with polyester gel coat to make parts and forms.  In large scale industry there are chopped mat guns that spray resin and chopped mat with air pressure to apply the material. These tools are available but can be costly for the small repair and build. The challenge with polyester resin is its cure time when trying to saturate highly textured materials like chopped strand mat. Even in manufacturing with a chop gun, that chops fiberglass into strand with resin and sprays it on the surface will always have holes, voids, air bubbles and delaminations right from the factory. Polyester resin does not have a long enough cure time to deair the materials, which is where epoxy and vinylester are better materials. The best way to use this material is in small batches with the minimum catalyst required for your air temperature and humidity, which is 1% v/v at 21 degrees Celsius with low humidity. Lower temperatures will require more catalyst, which makes for shorter pot life and faster kick times. This will lead to less saturation of the cloth, even if you apply resin to the surface before applying the chopped strand mat (wet lay up). Small volumes of catalysed resin will have the best chance at saturating the chopped strand mat. The aluminum rollers allow the builder to move faster on the surface but there will always be texture left by the rollers that will have to be filled with more resin. Brush application is also very common, but this is a slower spreading of the resin trying to push the resin through the mat, which is always a race against the pot life of the catalysed resin. With polyester resin, we do recommend multiple layers of fiberglass and resin. The reason for this is the many layers share load and flex, where one layer may crack and delaminate. It is always best to remove all of the unstable fiberglass and apply new resin and fiberglass versus just applying more layers over the top of cracks without removing the damaged area. Once again bondo body filling over cracks will not stop the cracks from weeping and continuing to crack, so applying multiple layers of cloth will strengthen and the top finishes will last longer.

    Cost per volume is the next question, with surprising numbers against the use of polyester resin when we calculate saturated surface area against the cost of the resin. As we mentioned quickly in the saturation thickness above, the polyester will saturate 51.3 square feet per gallon (3.74 litres) at 1/32 inch thickness. This is not enough volume to saturate chopped stand mat. A gallon of polyester through a 1/8 inch lamination (thick enough to saturate one layer of mat) would be 12.8 square feet per gallon on a non porous surface. We recommend a 3/16 inch lamination of polyester resin and gel coat as the minimum for strength in the marine, outdoor, patio, RV, hot tub, tub and shower applications. The gallon of resin and catalyst from our store is $83 per gallon with catalyst (Bondo Brand Fiberglass Resin is $89.99 per gallon), but your area of coverage is minimal. This is always what surprises builders that are repairing for the first time, because they end up purchasing multiple pails of resin and catalyst for reasonably small repairs, like floors and transom laminations. A client has recently purchased three 5 gallon pails (18.9 litres x 3 = 56.7 litres of resin) to coat the topside of a deck. According to the estimated coverages that volume of resin will saturate approximately 192 square feet on a non porous surface of his 260 square foot exterior grade plywood deck with a layer of mat. The volume of resin and chopped strand purchased exceeds $817, not including gel coat.  Below we will compare this square footage to cost of epoxy, with the estimated cost of coverage of polyester approximately $7.50 per square foot including the cost of chopped strand mat.

    The next adhesive in common in the construction of water tanks, chemical tanks, gas tanks, pools, hot tubs and corrosive application is vinylester resin. Vinylester is catalysed with MEKP, just like polyester, but it is slower curing and higher density particles which is formulated for corrosive and chemical resistance. This resin is used in many industries, but meets a higher standard and better testing because it is used in chemical plants, shipping, oil and gas industry which are highly regulated.

    Vinylester is used in builds and repairs of pools, potable water tanks, sewage tanks and kitchen type applications with fiberglass cloths, chopped strand mat and veil depending on the requirements. This is a simple resin to use, but it is a purposed based resin system so it is less common in boat builds and automotive, so most builders are unaware of its existence. Vinylester is used in industry and by specific technicians, but it is retailer available for $200 a gallon. Vinylester can be used in mold, just like polyester and epoxy, but the mold release agent must accommodate bonding to vinylester. Vinylester type gel coats are available, but are used in industry so they may be expensive for your repair or build needs. Due to vinylesters slower cure time it offers better bond strength and deairing of materials when compared to polyester. This makes it better for underwater, fully submerged applications, including tanks, boat hulls, hot tubs and solvent tanks.

    Vinylesters application techniques are dependant on the lay up, but it can be hand applied, spray applied, vacuum formed in mold and roller applied. This resin is far more versatile than polyester in its applications, but generally we boil it down to solvent and corrosive resistance, so it is not commonly used in boat repairs. There are many canoe manufacturers like clipper, swift and hellman canoes that have vinylester lay ups with Kevlar and fiberglass construction. The manufacturers are using the benefits of a stronger tensile strength and more flexible resin with a more flexible fabric like Kevlar. This allows for fast construction times with a resin that meets the build and quality standards of these manufacturers.

    Epoxy resin offers a high level of durability, ease of application and high bond strength when compared to vinylester and polyester. Epoxy resin is divided into three broad categories and they are dependant on the type of project and application intended of the epoxy resin. The categories broadly are art resin (jewelry, small objects, clear resin, pigments), casting resin (thick application of clear resin with pigments) and laminating resin (fiberglass repairs, structural repairs, marine, bonding materials together). We will focus on laminating resins because we will have to compare with similar application techniques of polyester resin, which cannot be used for casting and is not transparent. There is some advertisement for clear casting polyester resin, but one would have to test it to see the transparency against what is commonly used for river tables and deep casting. As mentioned above polyester resins cure time is faster and the exotherm (heat generated from curing) so casting at depth (more than ¼ inch per pour) the resin generates too much heat and can catch fire, discolour from the heat, or distort the mold from the heat. Be aware that retailers that are specifically sold on Amazon may not be able to provide all of the technical information on the product, so a builder can be purchasing a resin that is branded for one industry (or example art resin) as marine grade. We are contacted multiple times a day with people purchasing resin from Canadian Tire (Bondo) or Amazon resins that have many different issues with the resin including expired product, resin instructions in the wrong ratio, casting resin sold as laminating resin, not UV stable resin sold as UV stable resin and laminating resins without technical data sheets to support product quality. Epoxy casting resins have very long cure times (around 7 days depending on the hardener) and can be poured at depth of 6 inches. We will focus on the attributes of a laminating epoxy resin coverages, saturation thicknesses and cost per square foot.

    West systems laminating epoxy resin 105 and 206 hardener is recommended by marine structural engineers and some aircraft structural engineers for the use in the marine, engineered beams, electric car manufacturing, composite repair including carbon fibre bicycles, car fenders and diffusers, etc. There are two main reasons that we recommend epoxy over polyester resin for most constructions, which is the ease of use and the overall strength and durability.

    Epoxy is a fixed ratio resin and hardener adhesive that when mixed correctly will always cure to ideal strength. In the strength terms, West System 105 and 206 has a tensile strength of 7,300 psi and a flexural strength of 11,800 psi. It is hard to compare certifiable data for polyester resins tensile and flexural strength but the technical data sheet of Polystar polyester resin is tensile strength of 9,400 and flexural strength of 18,000 psi which would make polyester more rigid than west system 105-206. Bondo Brand Polyester resin has a tensile strength of approximately 5,800 psi and a shear strength of approximately 600 psi (bondo tds sheet).  The adhesive strengths of the polyester and epoxy do not account for other additives, curing techniques or fiberglass included in the strength and flex testing. These are excellent strength and shear strength numbers for both products, but epoxy will have more lay up time and pot life which results in less air in the fiberglass and thinner layers of fiberglass.

    Epoxy is much easier to handle and use when saturating fiberglass and laminating plywoods. The epoxy working time will depend on the hardener, but for West System 105-206 the pot life is 25 minutes, recoat is 90 minutes and full cure is 10 hours. This means that when you mix the epoxy it will take longer to begin curing and allow the builder to saturate materials like fiberglass. The longer working time allows for the material to be spread smoothly and allows the fiberglass to deair, not holding bubbles or air pockets. The 90 minute recoat time means that multiple layers of fiberglass can be applied while the previous layer is still curing, which allows for large areas  to be fiberglassed with out sanding. Polyester is fully cured in 90 minutes, so large areas have to be sanded and acetone wiped before the next layer is applied. West System epoxy will make the fiberglass transparent when it has received enough to saturate, once again giving the builder clear visual aids for how the lamination is successful. Polyester is a cloudy brown or blue colour that is not transparent when saturating fiberglass cloth or chopped strand mat. Air bubbles can be hidden in the material that the builder is unaware of. Epoxy is spread onto fiberglass with a flat spreader or chip brush, but since the epoxy is saturating woven cloths, we do not need aluminum rollers to push air out of the cloth. Unless specifically stated on the product, chopped strand mat is not compatible with epoxy resin. Woven cloths are easier to cut and shape when compared to chopped strand mat, and the e-glass or s-glass has woven structure instead of random mat fibres. The best result for the builder is always to have simple materials and application techniques to get a thin and strong lamination.

    Epoxy is also recommended for any marine application where the wood is being clear finished. The fiberglass e-glass and epoxy resin if applied in the correct ways will be transparent. Cedar strip, mahogany, ash or natural materials that need to be clear epoxy is the solution. We commonly discuss this with builders of wood boats, wood turners, outdoor furniture, outdoor gazebo and wood signs, where the product will always be exposed to weather, winter and humidity. We have found that barrier coating with epoxy will greatly lengthen the life span of the outdoor furniture. The downside to all resin systems, including polyester and epoxy resins, need UV protection and stability from gel coat, marine paint or clear varnish. Polyurethane Marine z-spar varnishes are recommended over epoxy and fiberglass for marine outdoor applications, these varnishes are formulated for temperature change, high humidity and cold temperature. When clear finishing natural materials the best material is epoxy and clear polyurethane marine varnish.

    Epoxy also has excellent coverage when applied through fiberglass cloths, and the coverage is more than polyester. As mentioned above the saturation of a 1/8 lamination using polyester is 12.8 square feet per gallon. The coverage of West Systems 105-206 gallon size epoxy through 6 ounce fiberglass e-glass is 95 square feet per gallon. A gallon of polyester was $6.91 per square foot without chopped strand mat, West Systems epoxy is $2.78 per square foot without e-glass fiberglass cloth. Polyester saturation cost including chopped strand mat is $7.50 per square foot, Epoxy is $3.91 per square foot including fiberglass cloth. The numbers are staggeringly different when we start calculating according to square footage with fiberglass, versus comparing gallon to gallon pricing.

    Polyester and epoxy are required when reinforcing natural and composite materials, but choosing the correct resin and the overall budgets when using one resin system compared to another. This blog post is intended to fairly look at polyester and epoxy pricing, coverage and application techniques so that a builder can be educated when selecting a fiberglass product. There are always the factory standards for repairs, where polyester constructed products are repaired with polyester resin and gel coat. There is also epoxy repairs onto polyester constructed products, and generally for first time builders and higher durability we do recommend epoxy. The results of both products as well as the familiarity with a certain method will also be a factor in choosing resin, but for the metrics applied today in regard to cost per square foot and ease of application West Systems 105-206 epoxy is less expensive and easier to use than polyester resin.


    3M 265 TDS

    105-206-Epoxy-Resin.pdf (


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