How do you know if you are sanding into the fiberglass?

General Discussion
1

I started sanding my fill coat and am second guessing whether or not i am sanding into the glass.  Are there tell tale signs?

I'm using Poly on S-glass.  After the fill coat dried, i could see a cross hatch pattern in some areas but it didnt look like the glass, it looked like the resin was just taking the shape of the glass underneath and creating its own cross hatch pattern.  Is it possible that the resin is thick enough and followed the glass and dried with a cross hatch?  So i started sanding and now have an uneasy feeling.  i'm sanding by hand with 150 grit.

Where i sanded the lap line on the bottom, i  am definitely touching the the glass on the edges because they fibers dont disappear when i wet them.  I'm not seeing that in the flat areas.

Any sure ways to tell?

 

thanks

2

When you sand into the glass, the fibers appear dry and you can see the weave of the cloth.  Dry and white, stop sanding, apply hot coat resin.