Polyester resin may be thinned by adding a MAXIMUM of 15 parts of Styrene Monomer to 100 parts polyester. Check with your supplier.   The first coat of resin applied to wood can be thinned for deeper penetration.   It should not be necessary to thin laminating resin as this weakens the cured laminate.  Thinning will lengthen the surface cure time and will require more catalyst.

   For health reasons, there are now some "Low Styrene Emission Resins". They have a substitute for Styrene Monomer or a reduced quantity in their make up. These resins are quite different from high viscosity resins which can be thinned with Styrene Monomer.