Yes, you are going to have issues until you establish the correct parameters for success. I do resto's on 4-6 classic/show cars a year &, of course, the auto sheetmetal for floorpans & inside panels are the thinnest. You need to set up your work area & determine your attack mode.
1) Flux-core is not the optimum wire for these thin panels.... 023 solid w/C25 is generally the best. Since you are using .030, you may reduce burnthru going to .035.
2) You need to get some backing material to use behind the seams to act as a heat sink & reduce/eliminate burnthru. I use 1.5" alum. angle 1/8"T & have pieces from 4-12" depending on location & fit. You can use flat copper or a piece of very flattened copper pipe that will work also.
3) You will get best results with an overlap of the seams about 1/2". The weld joint/seam must be CLEAN. No rust, undercoating, or primer/paint.
4) Your success will come if you take some cutoff pieces of the sheetmetal you are using, make some backing strips, setup your joint/seam, & practice. You have to understand you are NOT going to be able to run any weld beads in length. TACKING.... that is the only way. Every couple inches & varying location to reduce/eliminate warpage & having a very wet towel to cool quickly. You also must understand that with flux-core, you are "pulling" the bead.....that means more penetration. With solid/C25, you "push" the bead.... less penetration & increased gun angle will flatten the bead & really helps eliminate burnthru.
5) Since you are outside, you need to set up a large tarp to shield, as much as possible, the wind condition if you go to solid/gas.
Ok.... I have provided some solutions. Now you need to sit back, look at your overall work environment, determine how to eliminate the issues one step at a time. It is not hard.....don't make it hard. Solve the problems with thought & planning for your best approach. It will come to fruition if YOU want it to. If you get MAD......STOP! You'll never get it done once frustration/anger sets in. Go get a cold beer & take a half hour to "cool off". BTW.....there is nothing wrong or being "overpowered with your 187 for this kind of welding. It will do nicely if you understand your parameters & technique for success.
Complete weld/mach./fab shop
"A man's word is his honor...without honor, there is nothing."
"Words are like bullets.... once they leave your muzzle, you cannot get them back."
"I have no hesitation to kill nor reservation to die for the American Flag & the US Constitution."