Epoxy turning dark

[Aclouston]

Kent Ashton recently discussed a technique using an alu. bar to reveal high spots before/during finishing. I see older projects turning really dark brown after a while, especially on the fus. sides, making it probably a bit more difficult to use this rubbing technique. Its not just the foam as I have seen wings also turning really dark. Some builders say that a clearer epoxy makes it easier to evaluate the amount left in glass.( about air specks, I don't recall). With all other properties beeing  +/- equal, which epoxy; Aeropoxy, Mgs (or others) yields a light clear finish during layup? Which resist time better and stays clearer? Is the darkening a sign of anything other than time?

[Kent Ashton]

4 hours ago, Aclouston said:

Kent Ashton recently discussed a technique using an alu. bar to reveal high spots before/during finishing. I see older projects turning really dark brown after a while, especially on the fus. sides, making it probably a bit more difficult to use this rubbing technique. Its not just the foam as I have seen wings also turning really dark. Some builders say that a clearer epoxy makes it easier to evaluate the amount left in glass.( about air specks, I don't recall). With all other properties beeing  +/- equal, which epoxy; Aeropoxy, Mgs (or others) yields a light clear finish during layup? Which resist time better and stays clearer? Is the darkening a sign of anything other than time?

Parts built with EZ-poxy will darken over time but that is pretty normal.  MGS does not seem to do that.  Not sure about the other epoxies.  In any case it is pretty easy to see of the layup was properly wetted out with a good magnifying glass or jeweler's loupe.  If the glass threads are bridging or threads are standing out,  it was dry.  Too dry?  Depends on what parts and how much.  I wouldn't care if parts of a fuselage tube was a little dry.  If the layup has the appearance of standing liquid, it might not have been squeegeed enough.    I would use whichever epoxy is approved and the cheapest and does not give you an allergy.  I had no problems with EZ-poxy; MGS gave me a rash.   A fiberglass layup has to stay in the sun for many moons to deteriorate.

My suggestion for the aluminum bar rub is for when the filling is mostly finished and the surface is a light-colored micro or west Microlite filler that will show smudges from the aluminum oxide on the high points.