Hi Netters. Polyester stinks. I mean it has a terrible odour.Epoxy is not nearly as bad. Polyester will adhere to glass cloth and urethane foam just like epoxy will. Polyester will not last as long on top of wood that is exposed to sun and rain as will epoxy. Water tends to diffuse through polyester unless the top or outside coat is a polyester jell coat. Both require protection from the ultra violet of sun light using special paint ingredients designed for this purpose. The surface penetration of wood with both polyester and epoxy depends on the fluidity of the applied resin. The pot life of polyester is highly dictated by air humidity. Temperature is a factor in the pot life of both polyester and epoxy. The catalyst in polyester can accelerate the cure rate if it is used in higher amounts in polyester, while using too much catalyst in epoxy is a no-no. It changes the characteristics of the cured product. Epoxy has the advantage of the use of different types of catalytic hardeners to control pot life, hardness, flexibility and the unimaginable ability to stick very well to wet wood if and only, you use the Versamid 140 hardener.The ratio of 2 volumes Shell 828 and 1 volume Versamid 140. Such a mix is very slow curing , like 12 to 24 hours depending on temperature, but the results are excellent. The Gougeon Brothers book on boat building with epoxy is very good and applies perfectly to our airplanes. Never apply polyester resin to polyester foam. The results are fast and disappointing. The foam will dissolve and vanish before your eyes. This is why if you want to build a KR with polyester you must use only urethane foams. When polyester cures a waxy by-product of the cure migrates to the surface. This waxy surface prevents the next layer of polyester from bonding properly to it. The surface must be sanded and cleaned first. Never try to put polyester over epoxy.It will not hold. Similarly never put epoxy over even sanded polyester, it will seem to hold at first but eventually it will fail. Maybe next year at 10,000 feet? The final weight of a fibreglassed project will depend on the skill of the applier. You can make epoxy just as heavy as polyester buy applying too much. Personally I like epoxy the best. You can control the viscosity by your choice of resin which is best different for glue joints (more viscous) or for laminating glass or carbon fibre (use more fluid type). You can add thickening agents to the epoxy if desired. The polyester is easier to colour by adding a pigment that is designed for polyester, to the mix. The pigments for epoxy are different than that for polyester. It is more expensive. the pigments are not interchangeable. You can add finely powdered aluminum dust to an epoxy mix. It will provide U.V. protection. It will hinder the passage of radio waves. (A point that I misses on my construction.) This is an experimental hobby. Never believe every thing that you read , it may not be the final answer (even mine too). Experiment first yourself. Finally know that Epoxy is carcinogenic. Heed my warning on this. My neighbour did not. He used it as he would polyester. He washed the epoxy off his hands with acetone. This sent the epoxy through the skin and into his blood stream. 2 years later he died of lung cancer. Keep it off your skin. Use tygel (blue plastic) gloves at all times. Avoid the fumes and smell of epoxy. The same goes for the sanding dust but for a different reason.Once epoxy has cured it is inert. Sanded epoxy fibreglass has bit of glass in it . silicosis in the lungs can result. Good luck to all. Regards, Harold Woods, Orillia, ON. Canada haroldwo...@rogers.com
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