Prep Sol contains naphtha. I use Wyndex, original formula only, then naphtha, then tack cloth. Don't touch the surface, your hands contain oil. Prep Sol is probably the absolutely correct product, but, if you are at all budget minded, naphtha is much less expensive and from what I can tell works just as well. FYI, I have tried both. Also, using either for finish sanding, (final) with say, 2500 grit, it works much better that soapy water.