Our 2009 4Runner has been insanely reliable and I've kept the bottom oil-undercoated for years and it looks to be in perfect shape. However, today I noticed rust coming up through the top of the driver's-side door sill. What's the best way to fix this? The car looks to be in mint condition other than this spot of rust. Or is this a bellwether that portends a lot more rust coming up, and it's not worth fixing?