On this colour i have found that it isn't necessary to paint the tubes Neil, as it is rather dense ...
Jim,
I phrased my question cautiously as I don't know Terry or his experience but mainly because I didn't know whether the lines I can see along the blank are the tube or a lighting reflection from the photography. However, I find that whether you can physically see the tube or not, painting the tube deepens the colour of the acrylic, I do it as a matter of course. I wondered whether it would open up a debate on who paints what with what, I know some use a Q tip to paint the inside of the blank (which I think is a waste of time), as well as paint the tubes, and what do you paint the tubes with.
I use Halfords primer spray on the basis that it designed to be overcoated, adhere to metal, dries quickly and doesn't react with epoxy.