I have used Simtec/Prestec 2381 and Poly-Lux which I believe are
essentially the same. Simtec will add flex agent to any of these and
to their primer-surfacer, I believe. In recent years, I have always
had the flex agent added but I have no data on its efficacy over long
periods of time nor have I done a total refinish with it - and hope to
never again. As JJ has said repeatedly, it is brutal, messy, bad-for-
you work and barely worth what one gets paid to do such labor. I know
several pilots who stripped and/or refinished their own ships. The
next time they needed such work, they got out their checkbook.

A colleague has used the Ferro (spelling?) product and is quite happy
with it. He also mixes several different brands to meet his viscosity
needs.

Real-time experimentation is tough to come by. No one I know is
willing to gamble this much money and work. They tend to stick to
known, materials, understandably.

My observations suggest that the critical factors in making a finish
last a 1) Keep the thing dry, clean, waxed and buffed. Crazing is
a shrinkage phenomena. Keep the finish "hydrated" (not water) and
it will not shrink. 2) Keep the finish thin and definitely within the
paint manufacturer's tolerances . Thick coatings tend to mechanically
crack far more than thinner ones and craze as the trapped solvents
evaporate. This is true with both polyester and PU coatings and their
primer/surfacers. The latter phenomena has come back to haunt me
in the past and is a very, very common problem in the refinished ships
I have seen. Not only is the finish more likely to crack/craze, it
makes for a heavy glider.