To obtain the subject composition capable of bonding a polysulfide- based sealing material in sufficient bond strength to a slightly bondable material to be bonded such as a fluorine-based or an acrylic electrodeposition-based coated aluminum sash, by adding specific amounts of a chlorinated rubber and a catalytic component to a polyisocyanate.

This primer composition for a polysulfide-based sealing material is obtained by blending (A) 100 pts.wt. of a polyisocyanate such as tolylene diisocyanate, hexamethylene diisocyanate or isophorone diisocyanate with (B) 80-250 pts.wt of a chlorinated rubber, (C) 0.5-2.5 pts.wt. of a catalytic component such as N,N,N',N'-tetramethylpropylenediamine or a dibutyltin oxide-based compound and optionally further (D) a silane coupling agent such as γ- glycidoxypropyltrimethoxysilane and (E) a solvent such as ethyl acetate.