Naphtha can be used to thin varnish or oil based paint the same as mineral spirits. Naphtha is a faster drying solvent that is recommended for spray applications so runs are not as much of a problem. To me it has a cleaner feel to it if you were using it as a lubricant for sanding. For the most part I use naphtha to tell whether a existing paint in a house is latex or oil based paint. If you put some on a rag and rub the paint and it disolves enough of it that it comes off on the rag it's latex paint. The solvent won't do anything to oil based paint.is naptha better than mineral spirits? and why ? I am going to be using shellac for the finish on by crib as soon as it gets less humid here..