Sorry gentlemen. I am the moderator. By mistake, while I wanted to cancel part of my answer, apparently I cancelled everything.
The question was whether the AI would be happy to hydrotest an item painted. The person making the question appeared surprised of this possiblity.
Dirk Koelbl answered that he would not be HAPPY, but he accepts (see UG-99(k) unless the item is not in lethal service.
(k) Vessels, except for those in lethal service, may be
painted or otherwise coated either internally or externally,
and may be lined internally, prior to the pressure test.
However, the user is cautioned that such painting /
coating /lining may mask leaks that would otherwise have
been detected during the pressure test.

My answer was that I am not HAPPY but I have no concern. Paint does not hold pressure. I would ask to keep the pressure a bit longer. Actually I use to ask to keep the hydrostatic pressure for one hour just to leave the time the water to pass through any porosity. Attention to reinforcing pad: in this case the hydrostatic pressure should be maintained to give the water the time to pass through any porosity in weld nozzle/shell, fill the gap between the pad and vessel's shell and eventually drop out from the vent hole. It happened only once in my life: it was from three manhole openings on a 9% Ni criogenic service column. The safety board inspector had accepted it already.
Mauro

Mauro,
Maybe it's just my insane fantasy, but imagine the situation:
1 There is a porosity in a weld, which causes a leak
2 The porosity is covered with paint, no leak during hydro
3 The service is high temperature, the paint deteriorates after a time
4 The leak opens
5 The medium is a flammable gas, the vessel is in a poorly vented room
6 Spark!

I would say, ask the manufacturer whether it has been agreed with the user to paint before hydro. Just for their safety and peace of mind.

The Code provides for:
"However, the user is cautioned that such painting /
coating /lining may mask leaks that would otherwise have
been detected during the pressure test."

However I am convinced that paint (light primer, not heavy finish) cannot mask leaks. Konrad, I you continue with your assumptioin....water does not pass through fine porosity whilst gas or light HC yes, the result could be the same cathastrophic.
Mauro

Mauro, believe me, a decent epoxy or modified silicon paint, with strengthening additives like glass fiber and after proper curing, can withstand a lot of pressure. A paint can be very penetrative, it can fill the whole porosity so no pressure on earth will remove it. But after time, when the temperature and medium comes, it can decompose and start leaking. The AI's role is to make people aware. Which I've never seen, anyway...

The AWS Welding Journal reported in January 1994 on testing conducted to see whether paint could mask leaks or not. The tests showed that epoxy paint could withstand 2425 psig over a 0.0135" hole. Their recommendation was to not paint before hydrotesting.