You have to be resourceful and use a tool that will snake in behind the weld seam to give you the resistance from behind. When working a dent or weld, you shouldn't need to do any tapping from the backside. On a large dent, you can, but for the most part, you use the mass of the dolly to provide the resistance needed when tapping the panel from the face side. This will cause the metal to move outward, toward you as you hammer on it. In this case, if you can get something behind there that will resist the blows, the metal will move where you want it. Use your 3" cut-off wheel to remove all the proud weld bead except for just a few thousandths. The weld (tack) should only be very slightly above the surface of the surrounding metal. Place a dolly on the back side and gently tap with a fairly flat faced hammer, or better yet, a slapper. You will do this only on the tack welds. DO NOT work outside of the heat affected zone, only the tacks. In doing so, you will find that the sunken metal will quickly rise toward you and the slightly proud tack will be almost eliminated, as you will be cold forging it into the surrounding metal. This will make for a very strong weld seam. In fact, if it's done properly, it will just as strong as if there were never a joint there. It will be as one solid piece of sheetmetal. This cannot be accomplished with lap welding and especially not with adhesives. The adhesives on the market today are great, in their place, but this isn't one of them. I can assure you, that you will see the seam if you go the glue route. It may not show up immediately, but it will show up. Too many variables to deal with. Butt welding is the only way to fly when replacing exterior body panels if you want a high quality repair. All other methods are sub standard. This will get you started. If you get into any more trouble, let us know. Also, read the "Metalfinishing, Let's talk about it" thread for more info. The message I posted on mig welding the '39 Ford rear fender should give you the information needed to do this.
Metalshaping & Kustom Paint