SkipGundlach wrote:

I seem to have developed a sensitiviy/allergy to West System epoxy
sanding dust, or perhaps fiberglass.

Over the time we were working on the bottom of Flying Pig (now
launched, hooray!), my left arm, one day, itched uncontrollably. All
the usual itching relief failed: hydrocortisone cream, steroid
lotion/ointment, spray topical anesthetic (Lanacaine, e.g.) and the
like.

You've done most of the basics. The main solution to a contact allergy
such as you're describing is avoidance of contact. Gloves are a given.
Someone mentioned latex allergy. That's a possibility. Nitrile
gloves are pretty standard in medical settings to avoid that. You can
double glove for protection against torn gloves. Also a barrier coat
of some other sort. Desitin cream is an option. For treating the rash
after it develops, steroid creams like you mentioned, (hydrocortisone)
and antihistamines (e.g. diphenhydramine (Benadryl)). By prescription
there are steroid creams that are many times more powerful than OTC
cortisone. Old antihistamines like Benadryl are generally more
effective than newer non-sedating antihistamines such as Claritin,
though, if Claritin works, great. Standard antihistamines like
Benadryl are H2 blockers. Adding an H1 blocker such as the ones
normally used to reduce stomach acid, (Tagamet, Zantac, Pepcid and
their generics) may give extra anti-itch control. Systemic steroids,
e.g. predisone, for a short course should settle down a major flare-up
but are more problematic in long term use. For that matter, even the
OTC meds have their risks. Again, avoidance of contact is most
important.