I understand that the catalyst for Schwabbellack contained styrene and
that if a finish or repair was not thoroughly post-cured, then the
slightly lower crosslinking density allowed the styrene to migrate to the
surface over time...and styrene is yellow.
Post-curing through localized external heating or cycling the whole
structure to progressively higher temperatures increases the cross-
linking density. (Done properly on a major repair or new-build, this
can substantially eliminate polyester gel shrinkage over time and
largely avoid the need for refinishing. Provided it's not finished in T35
which is not UV-tolerant.)
With Schwabbellack the post-curing 'locked' the styrene and prevented
discoloration.
A chemist would be able to explain better and more authoritatively. Or
Wikipedia !
It's cosmetic only though. I had a new-build LS6 (one of the very last
from Schneider) that had yellow patches where lots of gel had been
used. Went like a dream.
Akso-Noble stopped producing Schwabbellack some years ago but
other polyester finishes are likely to have the same chemicals.
It can be really difficult to see how discolored a finish is - need
daylight but not sunlight and a reference colored surface.