Joining copper

Try using a slight reducing flame while keeping the filler rod with-in the flame while welding using low heat. Base metal and filler should be SS wire brushed clean and if thin at the weld joint use a clean SS block on the backside.

Is the CU base metal and filler from the same CU mix?

I've never tried borax/alcohol as a flux on CU but it should work well.

What does it do when you try to weld with oxy/acet?

T_Bone

Try using a lower flame temperature, about 1/2 of what your using now. Take a scrap piece of wire and flaten, then clean, then try running a bead using your filler wire. When your flame temp is correct it will have a molten puddle like Tig welding puddle.

Your filler rod is too large of diameter. Try some 18/20/22ga solid telephone/thermostat or one strand of multi-strand wire for filler.

If too small of filler rod you will need to add alot of fill too fast, then it would be time to increase filler rod diameter.

If too large then the filler won't be liquid when added to the molten puddle thus sticking just before entering the weld puddle. This leads one to believe that there heat input is too low so they add more heat thats causes the base metal to be over heated.

This would be true for any welding process and very noticeable with gas or Tig welding.

Remove the filler rod oxides with emery paper.

T_Bone