Abstract: A phenol-formaldehyde resin composition and formulating process is described which is especially applicable for use as either a heat cured or as a room temperature curing binder for sand in the production of molds and cores for the foundry industry. By the use of an alkali metal sulfite such as Na.sub.2 S.sub.2 O.sub.4 (sodium hydrosulfite) NaHSO.sub.3 or Na.sub.2 S.sub.2 O.sub.3 in the latter stages of resin formulation of the resole, a final product is obtained with excellent "through cure" characteristics and a high strength. The final cure of the phenol-formaldehyde resin or other acid catalyzed resins such as furan-formaldehyde is produced by a benzene or toluene sulfonic acid catalyst that includes a quantity of hydrofluoric or hydrofluorosilicic acid to thereby give additional binding strength when used for sand.