I'm not a bath expert (other than using one
) but i do have 30yrs of grp experience (mostly making tooling for general grp mouldings, boats, windturbines etc), and i can say that in all that time i've NEVER seen or heard of this before... :shock:
With just your fingertips running
lightly over the surface can you feel anything.?
With a tip of a fingernail running
lightly over the surface can you feel anything.?
Its difficult to tell from the picture, but choosing one of the larger areas of discolouration try pressing it with your finger and see if it 'gives' under pressure (probably won't, i'm just curious).
I'm sure i'm right in saying that 'fibreglass' baths used to have a gel coat finish when they first came on the market. Nowadays they tend to be an acrylic sheet which is heated then vacuumed into shape, fibreglass is then applied (sprayed) onto the back. This then NEEDS to be consolidated to increase its strength and to eliminate any air pockets. Regardless of cost my only guess is that this has been very poorly done. And as the acrylic can be very thin after the vacuum process, moisture(?) has over time crept back up to the acrylic and mould has set in, hence the dark patches you can see.? Can you easily remove the bath panel and check for any signs of damp/mould to the bottom of the tub.?
Do you know the manufacturer of the bath.? or at least where you bought it. I know you said its been in for 9yrs but it wouldn't hurt to make a few inquiries as to why yours has done it.... could be they've had others too.
Nick