OMG! Don't use pan scourers on GRP!
it takes a huge amount of effort to get back to a smooth finish afterwards.
There is a non-abrasive solution to hull staining called Y10. It's an acidic compound and needs to be sponged off with lots of water within about 10 minutes of applying it. Then treat the hull with a quality waxing compound as the surface will be really, really porous aftwerwards. You can achieve a finish that's as good as when the hull was pulled from the mould with this method. Y10 will remove staining, rust marks, and is widely used by yotties.
You can purchase Y10 from most good chandlers such as Shamrock Quay.
I've learned these techniques over the last couple of years from timboli and Jonny Fuller and they were well worth listening to!