Don't grind too much. Rust isn't always a nice, thin, even layer that you can take off. If you grind too much trying to take it all off you'll end up with nice shiny steel that is too thin.
Some people use acids and rust removers, rust converters etc and put on a nice coat of paint such a hamerite 'direct onto rust' or whatever it's called. I'm planning on giving that a go to see how it goes but not on inner sills where I can't keep an eye on it.
But, on bits I won't see again for many many years - especially if they're not bombarded by dust and grit - then I like a good wire brush attachment to take off all the loose stuff and then waxoyl. Brush that in well. I don't paint inner sill at all. Just waxoyl and lots of it.
I'm sure others will disagree and have their own methods. We're a bit like gardeners. We all grow our tomatoes differently and have our own methods. On a string, on a pole, feed, don't feed, pee in the compost (no i don't). But I bet my toms taste better than yours.
