[lgalla]

Alex good you found epoxy,The guys on the other thread are using laminating epoxy as well.Polyester and glass shrinks 1or2% making easy release from a mold.Epoxy does not shrink making mold release more difficult.In your case polyester may shrink and release from the steel.
In general polyester is not a good bonding agent.I doesn't stick to anything.Its strength comes from encapsulating the glass fibers Basically polyester fills in the spaces while epoxy glues things together.
Polyester shrinks for years.On 20 years old boats or less you can see the fiberglass print through and gelcoat cracking from stress .
Larry