A second layer of fiberglass mat is applied before a layer of core mat is applied. Core mat is a spongy cotton like material. It is available in many thicknesses. I used 2 mm. It is a great core material when weight is not a big concern. It absorbs a lot of resin, but it is a nice way to make a mold ridged without a lot of bulk. It also contours well, as does the glass mat.

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