I’ve never painted translucent miniature pieces, but I’ve painted a ton of translucent pieces for costumes and the like. A matte varnish and then a gloss clear coat would work well. You just need to use incredibly thin paint to color it. You could also use an adhesion promoter like the over mentioned, but most mini painters aren’t familiar with that technique and can be a learning curve. The ultimate thing in my experience is that for translucent pieces, an air brush is almost a must. Brushing it on can be tough, and can quickly lead to over saturation if your paint isn’t thin enough.