The method comprises joining a constituent, partly overlapping parts of a product. At least one of such parts has a protruding edge which bounds the overlap region and is pressed against the other part so that a self-locking friction joint is formed before the two parts are joined by welding. USE - Primarily in manufacture of open or closed hollow products requiring gas-tight or liquid-tight joints. ADVANTAGE - Drawbacks (weight increase, high cost, poor corrosion resistance) of known methods are eliminated.