Why does my weld have porosity?

Root causes

Gas shield interrupted — wind, empty bottle, loose fittings, or dirty/oily steel.

The fix sequence

• check gas first: flow, leaks, leaks at the regulator; shelter the arc; clean the joint
• Change one variable at a time and re-test on scrap
• Re-inspect the finished bead — chisel, brush, look

When to grind it out

Structural or pressure work with visible defects: grind back to sound metal and re-weld — never 'cap over' a defect and hope. For non-critical jobs, judge honestly: a cosmetic defect is a learning note; a fusion or crack defect is a redo.