Depending on how bad it is, you may be able to scrape / wire brush / blast / rust remove back to decent steel to simply repaint, however on both of mine the visible rust was the tip of the iceberg, the outer sill is a lightweight steel trim panel, behind it is the thick structural steel, it was all in poor condition and required cutting out and fresh steel welding back in, followed by primer then I chose Raptor to finish it. I drilled a couple of holes and used other existing holes to inject cavity wax into the inner chambers of the sill to stop it rusting out again.
Don't be tempted to use cavity wax if you think it's going to need welding shortly for obvious reasons!
This could also be the perfect opportunity to buy a new toy.... a nice MIG welder! Mine has paid for itself many times over.