I agree. The DB170 is great but it sure does stretch out of shape easy.
If nothing else, the mat helps to keep the shape of the material.

Lew Hodgett wrote:

Next time use a DB1208 and you will get the same results as the
DB1808 and
save a bit of weight. The glass mat sewn to the double bias strands
offer
good saturation and great adhesion and a superior water proofing layer.

I know of very few "pros" who use this stuff in fiberglass work but for
some reason it seems very popular with do it yourselfers.

snip

DB1208 is great for polyester but the mat does nothing but add
unnecessary weight when using epoxy.

Personally, I like DB170 for epoxy.

Lew