welding on negative pressure

I was having a hard time getting an airtight hull trully airtight when I discovered that hooking vacume to the hull while zapping a small but persistant leak is very effective. Just wondering if anyone is currently using this method?

 

The reverse method is more common - apply the exhaust of your vacuum to the hull to build Some pressure, but without blowing things out, apply soapy water to the outside in search for the leaking holes.

 

do you know how to press test?
when we get survey we always use manometer, which is simply a loop of clear tube half filled water, put 2psi in tank, and watch water go up in the second half of loop, should rise to 8 feet head, ok, then leave it settle, on cold day water will drop and hot will rise, but water should stay there, if drops and drops and drops , hey your tank it not tight, dont forget valves on the tank or water may get into it
for yachts in survey, same way they test a liferaft!!
no good just testing soap and water, this will find the big leaks, only manometer will be a true test
no charge for this info