Be very careful. Go at it nice and easy, CFR hand tool and don't over wet.
I had a BAD wickback from the same thing last year. It didn`t look that bad, cleaned up fine. The next morning I got the phone call, I went round, sure enough a
nasty stain that would not shift.
Then the grief started. The NCCA got involved and judged in my favor.
A hassle you can do without. Must admit I`m a bit parra about radiators now.

Good Luck and let us know how you get on.
Rab