best flux core wire to use

Butt welds break when they are mig welded. They will also crack. Butt weld two pieces together with a mig that you can bend and see how easy it breaks. Then try it with a tig weld.

Flux wire is for using in a wind storm in emergencies. Otherwise what would be the reason for having all that acid inside and out that is going to promote oxidation that leads to rust under your body work and paint. It is a no brainer. It is used as a quick repair of machinery or anything metal that breaks in the field. Not for body work unless you are going to sand blast both sides of your welds. Flux Starts rust.