Leak testing can sometimes be frustrating because it doesn't always work 100% accurately.
Sometimes worm holes or slag inclusions won't start leaking until the slag dissolves.
Sometimes, tiny holes or cracks are too small to allow the passage of fluids during leak and hydro testing, but will enlarge over time.
Also, leak testing should be completed before painting...
Not all leak testing agents are equal.
Diesel is thinner than water and will leak through a tiny hole that water will not, even under pressure, think penetrating oil.
However, diesel is not a good leak testing media in all cases do to health and safety concerns.
Compressed air will leak through a hole that is too small even for diesel to leak through, but hydro testing with compressed air is dangerous as it can cause an explosion that ejects shrapnel, in such a manner that fluids will not. An effective but safe, when controlled, leak test is to pressurize a system or compartment with just a couple psi of air and test the welds on the outside with soapy water. Bubbles = Leak.
A very accurate type of leak testing uses Helium as a test agent and a special detector to sniff leaks, because helium has a very small molecular size.
Glycol is an acceptable alternative in many cases as it is thinner than water, is easily dyed with fluorescent dye that helps detect leaks in dark cramped spaces through use of a ultraviolet light, has penetrating properties almost as good as diesel, and is safer to use.
You can use it in either structural or piping situations.
Careful though, ethylene glycol, or antifreeze (like you put in your radiator) is poisonous and release to the environment is EPA restricted.
There’s a lot more to it than this simple explanation.
Your company should have or develop a detailed procedure to regulate and document this process.

Tim