There is no indication of the spiral grouve. The mandrel it returned to the rotissorie. With the tube rotating slowly, it is sprayed with undercoat. I don't know now if this is really necessary. It is sanded off with carborundum paper.
Dust is blown off.
Now two pack Polyurethane paint is mixed and with the tube rotating slowly it is sprayed on. The painting is done late in the afternoon and the tube is left to rotate all night while the paint sets. Next morning the tube is lifted off and the mandrel and iron discs removed. I now had a tube wtth a smooth and shiny hard finish and no sign of the groove.