［Objective］To address issues such as sand adhesion，metal penetration，and cleaningdifficulties that commonly occur at the interface between sand molds with external chills and castings.［Methods］A suspension agent was prepared by composite modification of lithium based bentoniteusing a two-step method of "intercalation grafting" with bauxite white corundum as the composite refractory aggregate. The coating was formulated by combining a composite binder consisting of phenolic resin and polyvinyl butyral（PVB） with industrial ethanol as the solvent. Orthogonal experiments were carried out to systematically investigate the effects of the composite-modified suspending agent content，refractory aggregate gradation，and binder ratio on the coating properties. Theoptimal formulation was determined through range analysis and weight normalization.［Results］The results showed that the optimized coating achieved a 24 h suspension rate of 95%，a gas evolution value of 12. 6 mL/g，a brushing performance grade of Class Ⅰ，and excellent wear resistance. Production validation on 30-ton machine tool bed castings indicated that the chill surface coated with thedeveloped formulation exhibited a surface roughness（Ra）of 12.5μm，a sand adhesion area of less than 2%，and no obvious pores（Φ>2 mm）or metal penetration. Additionally，the surface cleaning time was significantly shortened，and the recovery rate of chills was greatly improved.［Conclusion］This study provided an effective solution for optimizing coatings used in the external chill chilling process.