Over the years I have been building ASME pressure Vessels I have been told that a Weld can not be painted until the vessel has passed hydro inspection as the welds must be visible during the test.
Can anyone point me to the section of code that covers this??

Ive now found the part of the code its UG-99 k. It says it can be painted prior to hydro. My question now is Would a AI inspector be happy with this???

Steve,

The answer is "no", the AI will not be HAPPY. But he will not have a reason to reject a vessel unless it is specified for "lethal" service.
The hydrotest is primarily not a test for tightness, but for structural integrity and major design mistakes.
The code informs the user that leaks could be masked by the paint.

Nonetheless, when leakage is detected, the AI can not accept the vessel.

DK

I am not HAPPY. But I have no concern. Paint cannot hold pressure. It is advisable to keep under pressure some time more. I use to ask the item to stay at the hydrostatic pressure for one hour, just to leave the time the water to pass through any porosity. Attention when there are reinforcing pads, specially if not air tested before: in this case leave the time for water to pass through any porosity in the corner weld, fill the gap between the vessel's shell and the reinforcing pad, before being noticed to drop through the vent hole. I experienced this leak only once in my life, but it happened and the previous safety board inspector had accepted the hydrostatic test already. It was a 9% Ni column criogenic service for ethilene plant.
Mauro

Thank you for your replies, your advice is extremely helpful.