The plaster mold had some imperfections, so I painted it with filler primer and wet sanded until it was very smooth. I packed the concrete into the form, same as before, but I made sure it was thicker all around. I decided not to put the rubber drain plug on the form this time, mainly because I couldn’t find it… I figured I’d just drill a hole if I couldn’t get the chair to release. And that’s exactly what happened.
After grinding the concrete flush to the form, and then blowing compressed air all around, it was clear that the concrete was stuck. Getting the first casting to release wasn’t a problem. I waxed the inside of the form with a mold release wax, but in my haste to cast the piece, I forgot to spray it with mold release as well. The primer was relatively fresh, and I’m not sure what happened exactly, but when concrete is curing, it heats up. The lack of mold release, and the heat during curing basically bonded the concrete to the primer, so the thing was stuck. Driving shims between the concrete and the form finally got the thing to release.. in 3 pieces.
So I’m back where I started, sanding off the primer, filling the imperfections with bondo this time, and smoothing everything out. A few coats of wax, mold release, 25 lbs. of concrete, a few hours time, and hopefully I’ll have another chair to play with.