Abstract An investigation was undertaken on a chemical modification of urushi paint using a small amount of primary amines as additives by taking advantage of their reactivity with urushiol, one of the main components of urushi liquid, at room temperature. The first modification method is related to an accelerated curing reaction of urushi paint, as a result of an enhanced formation of reversed micelles of urushiol. With this method, a large reduction in the curing time and curing at low humidity were achieved, and the light resistance of the cured film was also improved, as evaluated by an accelerated exposure test. The second modification method is based on the functionality of a compound linked to urushiol. Primary amine, having a water-repellent property, was synthesized, added to urushi liquid, and then cured. The resulting film demonstrated a good water-repellent property, and maintained this property even against degradation under accelerated exposure. These primary amines as additives undergo neither bleeding nor elution from urushi film, because they combine with urushiol. In addition, because these methods are based on a homogeneous mixing of additives and urushi liquid, they have advantages that neither large-scale plants nor equipment investment is required for their production.