While using old appliances as patch material is Ok.Your fab work is good.
You should be cleaning off ALL of the paint /baked on enamel,galvanize.That is on your repair panels.
If you take the time to do this you will notice your welds will look better and the machine will seem to actually run a bit hotter.

A hard disk grinding wheel on an angle grinder will remove the baked on enamel.Or a heay grit (36) soft pad
Grind both sides before you weld.The smoke from the heated paint will cause porosity (small pin holes) in your welds.

Good luck with your project