Rust

[06YarisRS]

Originally Posted by Spellino

I guess it means a trip to the welder

Probably the best bet. However, if you want to DIY, fiberglass offers an excellent, non-rusting, alternative to welding in steel. Any steel you weld in will still be subject to rusting - especially where welded - and it can be harder to ensure a water tight seam when welding metal in. Personally, if it were my car, and I was trying to save money, I would go the fiberglass route. In either case rust removal, and maybe conversion (phosphoric acid) would be sound preparation. Whichever approach you take, I would absolutely avoid putting any kind of rubberized (drying) undercoating over your repair. That is simply asking for water penetration and accelerated rusting.

Ex-x-Fire makes a great recommendation in rustproofing product. Fluid Film is an excellent and relatively long-lasting product.

Best of luck. And please document if you can.