I am presently putting together a security fence around a small container facility. I am using a rosin core solder and a home depot I am using a 30W-130W boost Weller gun and .032 Radio Shack electronics solder. Everything else is in the photo below. The #11 knife is just for scale.
I really don't have any experience doing the type of large section soldering that you are attempting, but I would suspect that the area is not getting enough heat only because it appears that the solder is not getting a chance to spread out before it cools down.
I usually apply a small amount of flux down the entire section that I am soldering and lay the iron right into it. It bubbles a little and I then lay the thin solder right onto the tip and draw it down to the joint. It appears that you are attempting to fill in the gaps like a weld in which case, you may have to fill in and grind down. However the smaller support pieces that make up the center of each section shouldn't need as much solder as shown in the picture. You may want to try a smaller point on your gun for the internal supports
What length spacing did you use between posts here?