Mat, when is it needed

On the few projects that I have had, I always used epoxy so I have a few questions about using mat with vinyl ester. It's my uderstanding that mat is used between layers of cloth to increase peel strength when polyester or vinyl ester is used. The binder in the mat is broken down by the resin, allowing the fibers to flow into the gaps between the two layers of cloth.
Here is where things get a little fuzzy in my mind. What about biaxial stitched material. It is basically two layers of unidirectional material with no mat inbetween, so would you need mat between layers of biaxial stitched material? What about very light weight smooth cloth like 3oz material and what about vacuum bagging. Both have little resin and gap between the layers. Has any peel strength testing been done to determine when mat is needed?

 

The mat fills gaps and enhances adhesion and peel resistance. If you fiberglass plywood, the mat goes against the wood. It would also go against cured laminate when installing bulkheads or bonding a deck to hull joint. My experience with thin cloth without mat is that is peels easily.

 

Is there any reason that you can't use epoxy resin with mat? Also besides the peel resistance, does mat add to puncture resistance and tensil strength?

 

The binder used in the mat will not dissolve in the epoxy resin.

 

Well you can use certain mat with epoxi. The problem is as mentioned the binder. So to have a mat with Epoxi it needs to be held together in another way.
You can get mat stiched on to a biax or triax and that can help some on print through etc. and also when fairing you won't damage the main laminate strength to much if you happen to sand into the fibers of the mat.