I have discovered a water spot on the inside of my AOI glass dome port. It must have gotten there from condensation. I have tried to remove it using a lens cleaning cloth and ROR lens cleaner. However, I think ROR is more for removing residual oil. Do you have any suggestions on what might work?

A drop of diluted citric acid (20%)

I used a diluted citric acid solution to try and romove a couple of water spots. It may have helped a little, but they have not gone away. I then used some lens cleaner and a soft lens cloth. I used more than a drop of 50% citric acid solution, which may be too strong. Should I rinse and dry the inside with distilled water or leave it as is?

Rinse and dry ASAP.

Even if it's a weak acid, don't use it at these concentrations.

Acid won't harm glass, it is alkaline solutions that can etch it. Certainly though whenever you clean glass ports do not let the water dry on there. It causes any salts in the water to concentrate, distilled water in theory should be free of salts but you may mobilize some salts whenever you rinse. The answer is to blow off most of the water (a blower bulb is good for this) then thoroughly dry - use a microfibre cloth.

This is a subject that crops up regularly and is a pain to resolve. As Chris explains, prevention is the best way!

Once the problem is there though I remember a range of solutions have been suggested; dilute citric acid or white vinegar; Coca Cola, Mothers glass cleaner, Brasso. I’m not sure anyone has nailed a clear winner. At the end of the day, re-glassing is only the 100% solution I’ve heard of.

7 hours ago, Chris Ross said:

Acid won't harm glass, it is alkaline solutions that can etch it.

Yes of course, but I was told that several dome/ports are treated/coated on the inside.

14 minutes ago, Davide DB said:

Yes of course, but I was told that several dome/ports are treated/coated on the inside.

It would depend on the type of coating, but I expect it would require strong mineral acid to have any impact, magnesium fluoride is often used as a glass coating and it's reported to be quite resistant to acids. Searching showed that magnesium metal is treated with HF to provide a MgF coating which resists corrosion by acids, much more so than the bare metal.