Rusty cast-iron fireplace

[earwicga]

How do I clean it off without scratching the fireplace?

what i've always done is to take them to be professionally sandblasted
then treat it yourself with a specialist grate polish
last time i did this i used zebo but iirc i think this is discontinued now
hopefully someone will come along with a recommended substitute

Ta, I don't want to be spending a fortune on it so I hope so too!