I've used Humbrol paints on plastic ship models for 55 years and cannot recall any serious problems. But metallic gold enamel, number 16, is a bit of an issue: I can't get the paint to leave the brush and stick to the plastic, and it will not come off the brush in mineral turpentine. Same problem with Tamiya gold enamel. It is demoralising to say the least. What on earth am I doing wrong?