RE: Thinning epoxy with Acetone and adding filler?
The last can of acetone purchased happened to have printed on the label that a primary purpose of acetone is as epoxy thinner.
When rubbing alcohol is used as thinner, a large amount of the water that is included as an ingredient is trapped when the epoxy cures. Epoxy is thick enough that it takes the water considerable time to migrate to the surface, and the epoxy cures way before that is possible for all the water. The result is a somewhat porous surface and degraded epoxy structure. Not a perfect result. Good enough for gummit work however, if you're not too particular.
Acetone is produced and sold to thin epoxy.
Why are you seeing longer cure times? Most two-part catalytically cured products are affected by thinning. Heat is one of the things that affects the cure time, and anything that modifies the heat would affect the speed of cure. The cure is also a coming together of the molecules and having to bring another molecule into the combination would also affect it.
I noticed the West Systems epoxy I've got takes longer than 30 minutes without thinning. Also, the LHS 30 minute takes less. My guess is just different stuff does differently.
Epoxy is used in boating often because it is more flexible than fibreglass and lives longer under the pounding. I'd guess lots of epoxies don't ever wind up being completely rigid.