my duster has a ton of it,
Tip: I would put some rust converter on the drip rails before you apply new caulking just in case so they don't rust in the future.
Thin wire brush on a 4" angle grinder and it's 10 minutes a side tops. It'll come right out. I highly recommend Lord Fusor self leveling seam sealer. It very quality and it ain't cheap. Takes a special applicator gun with mixer tips, but the stuff flat out WORKS.
I used a paint stripper wheel, looks like a sea sponge but i s some sort of hard plastic...3M you can get em at wal mart, some of them are double disc I'd take them apart down to single and used that on the drip rail and window channels