If I'm doing acrylics / polyester (and sometimes even Corian) I may have to paint the tubes...and I prefer 'overkill' - insomuch that I firstly choose the blanks dominant colour (as the colour I'll paint the inside with) - or I'll mix some acrylic paint together to get a close colour match. I'll then key the brass tubes, then spraypaint (usually white) then use the colour mixed paint to overspray the tubes when the white coat has dried. - spraying is done with a standard single action airbrush. This gives me an even base colour finish. I'll leave to dry overnight before gluing in the tubes. I've looked at some of the powder coating videos on youtube and yep to powder coat them is impressive and relatively quick - but there's a lot of expense involved. For me the airbrush method works well enough and the airbrush can be be bought cheaply online as can an entire pallet of acrylic paints. If you go the powder coating route I've no doubt you'll get top notch durable results.....and if you want to sell your powder coated tubes there's a market for them.