I've never tried using mineral spirits, as noted it would take a long time to evaporate.
Here are some things I have learned using Naphtha:
1) The tear strength of the resulting mold will be very low, so be prepared to get one or perhaps two castings off of it, and use a lot of mold release.
2) I found that using an accelerator can get the RTV to cure much faster which will help keep the mixture homogenous when using anything more than 20% of Naphtha. The accelerator is tricky to measure since it will depend on both the volume of the RTV and the percentage of Naphtha. A few too many droplets will make the RTV start to cure before you can get it into the vacuum chamber.
3) After many trials of Naphtha and the same type of RTV, I made a plot in Excel of the percent reduction versus the volume percent of Naphtha used. Now I can simply look at the relationship I plotted and I am usually right where I wanted in terms of scaling.
4) Depending on the proportion of Naphtha used, the library life of the mold might be about a month or so, so don't set it aside and come back later, expecting to still get a good casting out of it.
5) I can get about 22% reduction with 105% Naphtha. Anything more will be distorted. I usually only go with 100% maximum.
6) A large percentage Naphtha/RTV mix will usually not cure in the areas where it is in direct contact with clay, so be prepared to cope with that. I usually have to wipe the mold clean to remove the snot that is left.
Naphtha is pretty dangerous stuff. I mix the mold in my vented spray booth, and then I let the mold dry out in my garage, which takes about a week (with 100% Naphtha used). Be sure to read all of the precautions on Naphtha. I tried the "NoVOCs" stuff shown in the video, but a trial of 100% NoVOCs didn't yield anywhere near the shrinkage I get with Naphtha.
