I find that 6011/6010 works the best for open root or filling in gaps, seems like there's lest slag to get in the way while welding, and is good for dirty rusty mild steel. 7014 is a low penetrating fast rod, but still strong, but I don't like it for filling gaps, slag get's in the way easier, 3/32 works good for tacking, and 3/16 for getting a lot of welding done, and lot less spatter with it. 7018 is a good strong penetrating rod, I like using 6010 to fill the gap, then if I want more strength with less passes, I put some 7018 on it, it will penetrate, and burn out hidden slag.
After reading though this and other threads, I think I'm the only one in this country who welds/makes things with discarded bed frames, and truck springs. This steel is very strong stuff, great for bracing up something, and if I can get it free, I'll take all I can get. The best way to tell if you need 6010, 6013 or 7014, 7018, is to but weld a piece of bed frame and or truck spring on one side with one pass, then whack it against something, it should snap right into if it was welded with a 60XX rod, and usually it will crack while welding it. With 70XX, you will have a harder time to break it.