How far up on the quarter did you weld? The farther you go up, the lower the crown of the panel. Welding in low crown panels, with the shrinking the metal experiences from the heat of welding, is quick to loose it's shape and turn into a floppy mess (oil can) as you describe. Leaving a gap between the panels as some suggest, only allows for more panel movement due to the shrinking effect of the welding heat and nothing to prevent it. Ever weld on either side of panel clamps only to find them difficult to remove as the panel has shrunk around them? Case in point of the gap allowing the panel movement. This in turn will compound the flattening of the panel crown/oil can results as you are seeing. Where the panel will expand slightly as it heats, the molecular structure of the metal will realign tighter as it cools, thus shrinking. As a long linear weld shrinks, it has a tendancy to gather/pull the metal immediately outside the weld. A bit farther out, the remaining panel will have a tendancy to resist this movement. In extreme cases, the area in between the two will create a wave in the panel, and many people will misread this movement as the panel stretching. As an example, take a piece of paper and lay flat on a table in front of you. Hold one corner down with one hand, and using the other hand, push the adjacent corner towards the one you are holding. Moving the corners together will simulate the linear shrinking of the weld, and the resulting wave in the paper that results is what happens to the adjacent panel that is resisting the movement.
My preference is to fit the panels with absolute tight joints (or to the best of your ability) to minimze this panel movement. When using a MIG, use weld dots skipping around every so many inches. Before getting too crazy, insure that you have a full penetration weld with a slight bit of weld proud on the rear side. If you still see two separate panels on the rear after tacking, you have insufficient weld penetration that, given grinding the welds from the outside to produce a flat panel, will likely fail down the road. Turn up the heat if you need more weld penetration. If this tends to produce blowouts, turn up the wire feed a bit, as more heat and not enough filler results in burning the surrounding panel.
Next, after each weld dot, grind the front and rear of each weld proud to just ABOVE that of panel surface. Then using your hammer and dolly, planish each weld to stretch it to overcome the shrinking it experienced during the cooling. Next, some will suggest to add dots in between until you have a solid weld (keep grinding and planishing after each weld..) but I would recommend to instead overlap each previous weld dot by approx 1/3 to 1/2 dia for the next weld placement, as this will eliminate the problem of missing or leaving a gap between any weld dots.
By planishing each weld dot as you go, it will minimize the shrinking effects from welding, helping to eliminate any oil can effects. Then, any metal bumping required afterwards in order to straighten out the panel should be minimal.
