The first weld that you tacked, you stopped and then added more on top a second later.. Typically if you didn't get full weld penetration at the onset, welding on top is "blobbing" and still does not provide full weld penetration, only a taller weld to clean up. If you look at the back side, despite more weld on the front side you can still see the panel seam on the back side. Our purpose in welding these panels together in any rust repair is to duplicate what was originally there. In order to do so, your welds must have full penetration so we can go back and dress the welds flat. Looking at the other welds, many do not show full weld penetration. I would take the machine off of autoset, just rotate the knob slightly counter clockwise enough that the blue light goes out. Now, turn the heat up from your 16 ga setting to about 14 ga. Try your tacks again, and you should be able to decrease the time of trigger pull. Basically, you want full weld penetration, which the increased heat and wire feed gets you, but less buildup for less cleanup, which a decreased trigger pull should get you. If you can practice this and get proficient at it, it will help out with your MIG welding.
Next, most of the panels we weld on the vehicle are in free air, not laying on a metal work bench, so I always recommend to have the weld seam also in free air. If you rotate your coupon so the weld is centered in the slot, this would provide that scenario. Your practice pieces should duplicate as best as possible the same conditions you have in welding on the vehicle, as this helps to get your machine and your techniques "dialed in" to the exact same conditions you will see in welding your new parts in.. A steel work bench acts as a heat sink, which could also be causing some of the weld penetration deficiency.
You should be able to weld in this panel without the use of excessive filler. Any weld on sheet metal is going to shrink, so first and foremost, look for access from the rear side to be able to use a hammer and dolly to add some stretch back in the weld and Heat Affected Zone (that which is blue in color) when choosing where to put your weld seam. Generally, on parts like the rocker, don't worry about cutting a part of it, use the entire part. Panels like the rocker that don't have access for planishing, you are better off using the entire part and using plug welds to attach, which will have less shrinking issues than that of a weld seam. So worry less about using the least amount of new metal as possible, and more concern with planishing access, or methods that provide less shrink. Ideally, with sufficient planishing of the weld and HAZ, one could effect this repair without any filler. Realistically, we all need a little bit, but I would suggest focusing on the weld and planishing process and leaving filler as a last resort. Also, pinholes or missed spots between welds is a weld process issue, don't fix a weld defect with filler, correct the weld process.
If you do need to add a seam, make it near a higher crown area or a crease, as these areas will help to minimize the warping effect. If your patch panel has a stepped edge for lapping under, cut it off and do a butt weld. Lapped seams are prone to causing ghost lines in your finished paint, as seen here in this wagon tailgate, the bottom was repaired and attached using a lapped seam:
Similarly, using sheet metal of different thicknesses may cause this same issue. Anything on the exterior of the vehicle should have repairs done in as close to the original thickness as possible. I would suggest to continue practicing the weld tacks (in free air), adjusting the heat and wire feed settings until you get full penetration weld with each tack before moving on to your good sheet metal... My tutorials link below also has quite a bit of info on welding sheetmetal….
