Assuming the outside of the bath is not subject to direct water contact, it is made of upvc, bare, clean, dry, sound and prepared as detailed in the technical data sheet you could consider Kolorbond Aquatek.

This can also adhere to many hard plastics, or grp composites, we would recommend to test on a small area for compatibility and adhesion prior to full application.

This can be mixed to many RAL, NCS or BS colours in a satin finish.