ya i was thinnking gas sheilded flux core. im going to be building about 3/4's of it inside then the rest has to be done outside

hi im new here and am going to be building a trailer here soon. im going to be building a 20' equipment trailer with 2 7K lbs axles. im makeing it out of 6" channel and 3" i beam. i got a miller 211 and i am wondering what would be the best wire for this job. i am running solid wire now bu...