There is two types of knowledge on paint to be honest.
I have the engineers knowledge off process and what codes to go for and depth etc.
And then you have what your looking for which is the industrial chemist production. Which is a very rare beast indeed.
In alot of cases the paint is more cosmetic than actually doing anything. In these cases it doesn't really matter what you do it will always work out. But then when it is actually there for a purpose it is very important to do it exactly as it says on the tin.
By not curing it you won't have bonded all the molecules together it will be sitting there a bit like compacted and dried sand. It will be able to support some loads and shear forces but won't be accept the load that it would if you had put some cement through it.
An example off the top of my head would be if you had fixed a patch on your roof and a seagull shat on it and you left it for a month. I would expect that the uncured patch would allow the acid to get to the metal underneith and destroy the paint, where as the cured patch you could wash it and you would have never known it was there.
BTW if you need more hardener if you get your hands on the COSH sheet for the hardener you can find out what catalyst they are using. From memory there are 3 types for polyurethane catalyst depending what enviroment you want it to cure in. Then you can just order a bottle of it.
I presume you have had a read of this?
Polyurethane - Wikipedia, the free encyclopedia