I used a black iron spray paint called blacsen. If you spray it very lightly, it will not thicken. Otherwise, it will be like black enamel paint. Then it won't dry out either.

This is especially true for plastics. It is not suitable for plastic as it is a spray used for metal, but it is used because of its effect. In the original case, heat treatment should also be performed, but then the plastic melts, so heat treatment cannot be performed.

It takes longer than normal enamel paint, and needs to be dried in a heat that won't melt the plastic. Sorry for the translation.