So your flange material was glass?  It certainly wouldn't be a reaction to glass.

The gelcoat looks very very thin so it almost looks like the gel has lifted, either due to trapped air or possibly the coupling coat pulling it away from the mould surface.  In thin sections like that, the gel probably isn't building up much heat compared to thicker sections.  Normally you would get the alligatoring effect if the gel wasn't cured enough before double gelling or coupling coating but i think your gel is so thin it hasn't done that.  Either that or you have managed to somehow trap a lot of air under the gel as you laid it.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Hi, after the first not successful mold and part (http://www.talkcomposites.com/Topic36543.aspx) I worked on the part to fix the issues in order to build a new mold.
This time the part popped from the mold almost by itself but... something has happened to the tooling gelcoat!
Please se pics. Last pic is the tooling gelcoat on the base glass, so I tend to exclude some interaction with the plug.
Any ideas?