Ok, this isn't for my car but I figured it could be relevent for someone doing bodywork hence in this section......

I've got an old bath that has angle iron reinforcement around the top. The bath is going to the tip but I want the steel. It looks like its got the GRP resin holding the steel on (solid white gunk thats also in the GRP matting).

What is the fastest / easiest way to get the steel away from the GRP? The GRP can be destroyed in my case.

If i'm going to cut the GRP - what sort of mask should I have?

Use a dust mask, and wear overalls/gloves, as the dust will make you very itchy (and you do not want to be breathing it in!). GRP is fairly easy to cut.... A hacksaw would be the controlled method. A small angle grinder/diamond wheel would do it a lot quicker! Failing that, a number 3 adjusting hammer (large!) applied liberally should also yield the desired results!

I must have my grumpy trousers on.

I wouldn't bother. Approx 5m Max of steel? Probably a tenner to buy new?
I just reckon you could take a while to extract it and get it clean enough to use. This is most unlike me, but unless you are desperate, (angle grinder with thin cutting blade, normal steel one works very well, clean up with wire brush in the grinder. Found hacksaw blades lose their edge quite fast with fg) but the amount of dust is horrendous. Decent mask. A foldflat one should suffice, proper rubber one better.decent glasses/goggles to keep dust out of your eyes.

another vote for why would you... it sounds far too much hassle for a bit of angle unless its something of a really unusual size or profile.

Your right but I hate waste & if I've got it I can use it