I don't know if I would consider myself a "welder", but I'm doing it and learning as I go.....experience is the best teacher, and all that.
I find when welding two different thicknesses of metal, it's better for me to begin the weld on the thicker section (16ga. in your case) and move to the thinner section (18ga. in your case).
It may be psychological, but that still works for me too!
Good luck!

~ Victor
1941 3/4-Ton Pickup (in process). Read about it in the DITY Gallery
1955 Grumman Kurbside "Doughboy" 235/3 on tree w/ OD
1957 3100 - moved on
1959 C4500 Short Bus "Magic Bus" - moved on
1959 G3800 1 Ton Dually "Chief" - moved on
1958 C4400 Viking "Thor" ~ moved on to fellow Bolter