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Haven't messed with paint thinner or acetone to thin it. Denatured alcohol works *really well* to thin it. I learned to thin it for fiberglassing model airplanes, and never looked back. I've been able to get it thin enough to airbrush, thick enough to fill, and anything in between. If you put it on with your thickener and decide you want a really smooth final finish, consider thinning it and applying it like you would paint. Guaranteed to be compatible with previous coats, and thinned it can produce a nice shine due to surface tension.
Methyl alcohol is a thinner for epoxy, if you want more options for applying it. I've had great success using it.
You mention the epoxy being thick and difficult to get smooth coats with. You might try thinning it with methyl (denatured) alcohol. At $20 per gallon it's cheap, and the epoxy paint flows better. Might save you some sanding time, and it cleans paont brushes really well.
https://en.m.wikipedia.org/wiki/Mars_Exploration_Rover will explain Spirit and Opportunity.
Rocketeer Cirrus X3 Rocketpack
3D Printed, Ultralight, 3-axis Modular Time-Lapse Motion Control System