Sorry to take so long to respond. It's been a busy week. Here's a quick Cliff's Notes version of it -
The two most common fiberglass cloths are mat & roving. Mat (aka CSM or chop strand) is usually used where the new glass bonds to an old surface, in places where irregular build up is needed & on the surface below the gel coat. Roving goes in the rest of the sandwich to build thickness & give biaxial strength. In many lay up schedules, mat & roving are alternated in successive layers.
Both mat & roving come in different weights. Roving has strands that are woven together in 2 different directions. Mat is a bunch of random strands that are held together with a weak "temporary adhesive" that is formulated to disintegrate when soaked in resin. This lets the random strands go where they need to go to fill in small irregularities & smooth out into a flat surface. This stuff also wets out real well.
The older versions of mat had an adhesive that did not dissolve properly in epoxy. They only worked as intended in polyester resin (the stuff that uses drops of MEKP hardener to ounces of resin). If you use mat, buy one that is epoxy compatible, or else you might have trouble getting it to wet out like you want it to.
Another option, that has become very popular, is to use 1708 composite cloth. Composite cloths come in different weights, but 1708 seems to be the popular one. This is a biaxial cloth with a layer of mat bonded to one side. If you get this, then you only need to buy one type of cloth. I did my motor mount repair the old school way, with alternating mat & roving, because that was what I found in the scrap bags at a local boat builder that week. He builds with epoxy, so I was pretty sure that I had the right kind of mat. Roving is roving. You don't need to worry about roving being compatible with epoxy. It's all good. 1708 is also all good.
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