Ever since midwestclay.com stopped selling the "Grey Sculpting Compound" to me, I've developed a matching hand-mixed formula using Premo & Super Sculpey.
1 of the 4 extruded bars that come in a 1 LB pack of Beige Super Sculpey
1/2 extruded bars of Premo White from the 2 OZ bar.
1/4 extruded bar of black Premo from 2 OZ bar.
This mix matches pretty closely the Polyform Grey Compound. This mix with premo gives the baked sculpt crack resistance and flexiblity.
I agree that all crack repairs have to wait until the final bake and cool-down and 24 hours cold before repairing hairline cracks by super gluing and shutting crack while it's warm. Big cracks get Aves after sanding and smoothing.
Wayne
www.figuresculpture.net