sm_jamieson wrote:
I got a reasonable quality fibreglass/acrylic bath. The more expensive
ones were either not a style I liked, or too expensive.
The base is entirely flat so good for showering and for supporting the
bath, but the sides flex a bit. I want to glue something like chipboard
onto the sides to improve this.
The underneath surface of the bath is uneven encapsulated fibreglass
matting.

I'd be far more inclined to buy some fibreglass mat and resin, and
stick that on. Brush on resin and hardener, plave the matting, stipple
on more resin 'till the mat is "wet", let it go off, done.