There are numerous ways to test paint cure but one of the most traditional is MEK rubs. However this is an often misused test.
MEK is methyl ethyl ketone which is a good solvent for a wide variety of polymers. It is a common ingredient in solvent paints. MEK is a highly volatile and highly flammable liquid.
It is a smaller molecule than xylene or mineral spirits and because of this it enters between the molecules of a cured coating. This actually causes the coating to swell and makes more space between the molecules.
Paint cure occurs when the molecules of a base polymer, such as a polyester or acrylic which consists of a long string of atoms connected together, are connected together by shorter molecules between them. You can think of it as a bunch of strings or spaghetti, which move easily and freely with a bunch of smaller pieces of string. When the paint cures they become tied together, the longer pieces with the shorter pieces of string. When enough shorter pieces of string bridge the gap between the longer pieces they can no longer move and become rigid. Then the paint is considered “cured”. The amount of cure is the amount of little strings which are tied to the big strings.
The way you measure paint cure is by counting the number of double MEK rubs the painted surface can pass. You actually count the rubs back and forth. One back and forth is one “rub”. Over the years the spec has changed slightly and some people use cheesecloth and some people use cotton and some people use a hammer to put a constant pressure on the surface. There is a variation between these techniques but the results do not vary greatly.
Explained simply, a cloth is wetted with MEK and rubbed with a slight to medium pressure across the surface of the paint. As the MEK evaporates it is remoistened so there is always some solvent on the cloth. The surface does not need to be flushed with the MEK it just needs to be dampened. When the paint is rubbed through to the first appearance of metal, the test stops. That is how many MEK rubs it takes to fail the test.
In the simplest terms the MEK causes the polymer at swell and as the cloth is dragged across the surface of the paint the chemical bonds, primarily the cross linking between the main polymers is broken by the abrasion. The more cross linking there is the more rubs will be required to break though the surface. A heavily crosslinked coating may take hundreds of rubs to get through the surface. Certain paints may be resistant to the test and it will not be a valid test on these paints. But most commonly encountered paints will respond very well to the test.
There used to be a rule of thumb in some circles, that 100 MEK rubs is a good measure of paint cure. Manufacturers recommendations should be followed, however, as some polymers may be different.
Generally, as a coating is heated and spends more time at a cure temperate it will become more cured and the MEK rubs will increase, the flexibility will decrease, the hardness will increase, and the corrosion protection will increase. This is not an absolute rule but it usually holds true. It is also usually true that a temperature and time will be reached where the polymers in the paint will start to degrade. At this point the MEK rubs will begin to decrease sometimes dramatically and there will be a detrimental change in the coating.
But you may sometimes under or over-cure a paint because you want to push its characteristics one way or another. An under-cured paint will usually be more flexible.