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I often use both the phenolic and the Microballoons . Properly mixed, both are quite strong with the Microballoons (I believe these are fiberglass) being somewhat stronger. Always use a slow epoxy, 30 minute or longer, and be sure to mix the two parts well before adding the balloons. Keep adding balloons until you get a mixture like very stiff peanut butter so that it will not run or sag after application. Either will sand quite easily, at least easy as compared to fiberglass or epoxy alone. While still uncurred, you can smooth it with your finger wetted with alcohol to minimize any sanding requirements. If you area covering with a light colored covering use the Microballoons as the phenolic are a dark brown and will show through most coverings while the Microballoons are a nice white and easily covered.