you just use a acid brush and paint the joint before you weld it .most of the time on thin tube i use no filler rod if light tight they come out good the solar flux just helps keep the grapes look on the back side of the joint with stainless steel on short parts i tack the pice then hold the torch and just turn the tubing with the speed i need to get the weld look the way i want one of them deal the more you do it the more you get the hang of it