Ron, I belive Vikki was referring to the factory seam sealer used where metal overlaps, such as where the floor pans meet the rockers or like you'll see where panels meet on the firewall. Seam sealer is like caulking you might use in your home, except it's used to seal moisture out of sheet metal connections. There are many automomotive seam sealers on the market today. The better ones require a mixing gun for two part cartridges. There are single component seam sealer that brush on. Most will be better than the products originally used your Firebird. If you do a flang repair, you want to seal the joint overlap using a seam sealer product so that water or moisture don't accumulate in the joint and cause rust or rot. Seam sealers by SEM, 3M, Lord fusion etc will seal a flange joint against moisture. A butt weld is finished with primer and paint without need for seam sealer.
A flange repair in your floor pan won't show until you look from underneath the car. Then it will be pretty obvious. It won't be noticed through the carpet as a bump. The panel will be thicker by about the extra thickness of sheet metal where the overlap is. You will see it from below, but you won't see it through the carpet. You will need a flange forming tool or a bead roller. A flange repair is like a tire patch and is not a restoration level repair. If you have exposed body patches to make, the floor repairs are a good place to hone your skills. On body panels a flang repair can show through the final paint. The flange is like an I-beam down the length of the sheet metal. It can show through like the ribs on an umbrella as it heats in the sun. You'll read in the link below that a flange repair cannot be metal finished to completion.
I thought I would pass along a couple of links to help you understand the process and benefits of doing a butt welded repair. The disadvantage is the greater welding skill required and the additional fitting time. Butt welded panels should be fit as close as you can get, edge to edge before welding them in.
What Randy Ferguson explains here is the process I am using for all my rust/rot repairs. His explanation is where I learned this type of repair. Teach yourself this process and you'll no longer worry about a hole here or there on your car. You can teach yourself the process with some practice. Just practice before you start repairs on your project car.
