Because the first one was written with the panels still on. Once trim and liners come off, the shop finds what the outside hid: a cracked bracket, a bent reinforcement, rust eating along a seam. In Ontario, the bill can't go more than a small margin past your written estimate unless you approve it. Quebec garages need your authorization for extra work too. Ask at drop-off who calls you, and get the new figure in writing.

Usually yes, once the shop says the paint has cured. Grinding and welding burn off the factory coating inside panels and seams, and fresh bare metal is exactly where salt gets started. Oil-spray rust-proofing is a common yearly booking in Ontario, Quebec and Atlantic Canada, and the oil creeps into seams a hard undercoating can't reach. Tell the rust-proofing shop which panels were repaired so they get a proper soaking.

Whatever the shop writes down on paint and workmanship, and many body shops cover their refinish work for as long as you own the car. Provincial law adds a floor under that. Ontario's Consumer Protection Act gives parts and labour a minimum warranty, and car repairs in Quebec carry a legal warranty. Neither one covers fresh damage or rust that starts somewhere else. Keep the invoice and ask whether the shop's own warranty transfers when you sell.

Depends where it is and how far it's gone. Bubbles on a rocker panel or the bottom of a door usually mean the metal is already thin from the inside, because salt works outward from the seams. Cut out, welded and sealed properly, that repair lasts. Rust on a frame rail, a subframe or a strut tower is a safety problem first, and on an older car it can cost more than the car is worth. Ask to see the metal they cut out.

In Ontario, a written-off vehicle gets a brand: salvage, irreparable or rebuilt. A salvage car needs a Structural Inspection Certificate before it can be branded rebuilt, and then a safety inspection before it can be plated again. British Columbia and Quebec use comparable brands. So if you buy back a write-off to fix it, budget for those inspections as well as the body work, and pick a shop that knows the process.