If you could post a picture or describe the split (how big, is it caused by rust, accident dammage, fatigue) a bit more it would help to offer some advice.
The best way is to cut the rust away to good metal and weld in a patch, then prime, skim with a little bit of filler if needed, and top coat. All very well if you can weld but filler on it's own is about as strong as a cheap digestive biscuit:I
