by Roger » Fri Dec 21, 2007 11:34 pm
Each has its strengths. The mat is more waterproof than the cloth, but the cloth has greater linear strength in two directions. The mat is rougher and thicker so is better for building up a void. Cloth creates a nicer finish sooner. Mat shapes better than cloth over irregular surfaces. Boat builders will alternate cloth and mat layers, so from the outisde in, they would spray gelcoat onto a waxed mold, then mat, then cloth, then mat, then cloth laid diagonally to the first layer. Cloth tends to show its weave is it is the layer directly below the gelcoat. Cloth is typically easier to work with.
Tell us what kind of damage you are repairing and where it is on your boat and we may be able to give you better direction as to both material and technique. For example going around tight radius corners typically calls for a particular weight of cloth laid on the bias.