We are currently using the HEPA filter 999-137 (HJ3100 fume extraction system) to filter fumes for mineral spirits. Is this the correct filter for that application?


No.  In fact you should NOT be using the HJ3100 to absorb any vapors from mineral spirits or other vapors that are combustible.  The Hakko HJ3100 fume extraction system does NOT incorporate a sealed motor assembly and therefore cannot be used to remove flammable fumes/vapors, also known as volatile organic compounds. A spark from the electronics/motor inside the unit may ignite combustible vapors.

The 999-137 filter was replaced with the A1586 Main Filter that is also compatible with the new Hakko FA-430 Fume Extraction unit.  The same applies as the FA-430 does not use a sealed motor assembly either.