_The Evening Post_, 17 May 1924:
TO REPAINT A BATH
When it is necessary to repaint a bath it should be rubbed all over with sandpaper or pumice-stone. After this apply two or three coats of bath enamel, which will adhere much belter when the surface is roughened. In order to preserve the paint when the bath is in use, be careful to turn on the cold tap before letting hot water run into it.
https://paperspast.natlib.govt.nz/newspapers/EP19240517.2.132