[BAM55]

What you have there is called shrinkage. Welding causes shrinkage sometimes very little sometimes a lot. Thats why we tack weld and take our time when welding sheetmetal. Now you have a HAZ (Heat Affected Zone) when welding its the area where the metal change color in and around the weld. That is the area causing the shrinkage. To fix this I would hammer on dolly. Since you have access to the back I grind both sides of the weld, place my dolly over the front of the HAZ and hammer from the back. Hammer on dolly is stretching the metal so you are trying correct the shrinkage. Experience panel bumpers can do this by placing the dolly on the back of the panel but its easier for a beginner to do the reverse.You can use a guide coat on the bare metal to see how close you are getting. Many cases you will not have access to hammer on the back of panel and thats when you will truly need to learn hammering on and off dolly. Oh and your dolly should closely match the shape of your panel. Do not beat the heck out of it trying to get it to move it takes patience.

Its hard to explain but I hope I helped.