welding on negative pressure

[tightfits]

While trying to seal up an airtight aluminum hull I discovered that hooking a vacume to the fitting used for air testing while zapping a persistant leak was very effective. Is this a common method?

[kmorin]

tightfits,
yep that's the easiest way to keep the [heated] air inside from expanding into the weld zone and perpetrating the porosity/leak problem- pull a vacuum.

Its just as easy to do with a cold wet rag, though. The cooling of an area of plate will usually cause the void to suck in air and all it takes is enough 'inflow' to pull some molten puddle in to freeze in the void, then close out the weld on top and most decks seal.

cheers,
kmorin