roving, cloth or mat?

[scosch]

I havent done fiberglass repair before but will need to do some this spring to eliminate an unused seacock and also to build up some backing discs for replacing other seacocks.
Ive read this site, Casey, West System and other FRP books and I seem to be missing the subtleties of the difference between fiberglass materials. I understand the grinding, cleaning, resin, etc processes, but Ive seen suggestions for laminating repairs using alternating mat and roving, cloth and roving, mat and cloth, pretty much all combos.
And the seacock backing plate building process from this site suggests using cloth and roving but no mat. Why?
Can someone elaborate/ improve on my basic understanding:

Mat- random fibers yields strength in all directions (used to keep cloth or roving from peeling as easily? faster build up too?)
Roving- heavy woven cloth (used cause its faster build than cloth? anything else?)
Cloth- lighter weight woven, stong in 2 directions (why alternated with mat? to add some structural definition or easier bends on curves?)

[sailingdog]

Cloth and Roving produce a laminate that is far stronger than you can make with matte. The longer continuous fibers give the laminate far greater strength than the short chopped fibers that are found in mat.

Mat is often used to build up laminate thickness quickly. It is also often used as an exterior layer on fiberglass layup to prevent or reduce the pattern print through, which can happen with cloth or roving.

Cloth and roving are both used in laminates, but the roving is far heavier and allows you to build thickness faster, but it isn't as flexible as cloth, so can't be used for complex shapes or curves.

For very complex or intricate shapes, very light weight fiberglass cloth is generally used to do the layup.

They do make cloth and roving in triaxial weave, with two layers at 45˚ to the main warp, as well as the more common biaxial weaves. The cloth or roving presents the greatest strength in the direction of the fibers.

BTW, some mat is held together with binders that are designed to dissolve in styrene, and really shouldn't be used with epoxy resins.

[scosch]

as usual, thanks sailingdog.
one final question. I use only cloth for laminating unless I want to build faster and then I could use the heavier roving in alternating layers?

[sailingdog]

No, you can use cloth or roving... however cloth will give you a smoother, fairer surface. It will also drape better and work in corners and curves better.

[danjarch]

Depending on where your using it. One trick is to make a quick chopped strand for build up. Take scraps of cloth and cut it up into about 1 inch or so squares, pull these apart into the fibers, you don't have to pull each fiber loss, just pull them quickly. Mix in epoxy and wood flour or silica and you can fill small patches really quick. Then cover the fill with cloth so you wind up with a cleaner look. Let it all cure and fare it in.

Does the same as mat but you don't have to buy a seperate roll of mat or roven.