Dcr, it would depend on where your frame is rusted as to whether you would change the integrity. Pictures would help. It is possible to cut out patch and grind an area to look original without causing a problem. A couple of things to keep in mind are the material used and also the welding methods. Use a mild steel plate or tubing section equal to or slightly greater than the frame thickness. Make sure the frame and metal used is clean of paint, rust, or grease well past the weld area. Use a mild steel rod. E-7018 will be best because of the nickel content. Use 70 s wire if tig welding or 70 series mig wire. T.I.G. welding would be best because it would allow you to use the least amount of heat and still get 100% penetration. The more heat you apply, such as when stick welding, the more the chance of overheating the metal. When you do that there will be an excess of carbon drawn into the weld and this can make it brittle (hence the nickel in the 70 series rods). The crystal structure of the metal will be permanently altered making it prone to cracking. Also avoid square corners if the patch is in the tubing. Radius the holes or do a diamond like cut so there will be diagonal lines instead of cutting 90 degrees to the frame. If the area to be repaired is in a reinforcing plate it won't be as critical. I wouldn't recommend brazing or oxy-acetylene welding on a frame. If you are an experienced welder, fine, but any heat applied to a frame is a critical operation as you can easily cause weak spots even if you scab plate it.