When learning about different finishes, some resources talk about the finish's drying time, such as this Zinsser Shellac.
The shellac dries to the touch in 15 minutes and can be re-coated in 45 minutes
Whereas others refer to a curing time, like the Real Milk Paint Tung Oil:
Pure Tung Oil will take 7 to 10 days for a minimum cure and 15 to 30 days for a full cure.
Is this the same thing? If not, what is the difference? Can the same finish have a curing time and a drying time?
While it's correct to say these terms can be used interchangeably they should not, the meanings when used technically are quite distinct.
The problem begins with the word drying being overused in product descriptions for paints and other coatings; that is, "drying" is used loosely and therefore will always be inaccurate in some instances. But the concept of paint or varnish "drying" as a catch-all for any process of hardening is far too entrenched to change.
Drying literally means drying, an actual drying process where at the simplest water is lost by evaporation. For other coatings it is merely a different solvent (or mixture of solvents) that is evaporating.
Curing on the other hand is less of a populist term and should have its correct use maintained so as not to muddy the understanding of its meaning.
Most paints and varnishes, as well as many lacquers, undergo both drying and curing processes. Drying is the initial phase, where the coating shrinks due to the loss of the solvent component. Curing is the second (usually much longer) phase where the coating changes physically and/or chemically; it may swell slightly during this process.
Where there is a chemical change during curing it can be components within the coating combining and reacting, or a reaction with oxygen (or water) from the atmosphere. An example of the latter occurs with oils such as linseed oil and tung oil, which undergo oxidative polymerisation, changing from oils into a type of natural polymer by reacting with oxygen.
Where a coating only has a drying process, such as with watercolour paint or shellac, it is chemically unchanged and therefore remains soluble in the original solvent. This is why even decades later watercolours are water-soluble, and why shellac can be re-dissolved in alcohol.
When a coating cures its chemistry changes from that of its original liquid form, so that for example a paint that was soluble in mineral spirits (white spirit in the UK) at the time of application will, after curing, no longer be soluble in it.
Drying is the process of a solvent being removed from a finish by evaporation. For example, water in acrylics or alcohol in shellac. Typically drying processes can be reversed by adding the solvent back to the dried composition, being a physical change.
Curing is the process of some chemical reaction finishing (where the polymers become cross-linked thus harder). Examples would be two part epoxy, oil paints or tung oil. Some are reacting with the oxygen in the air (like tung oil) and others with mixed components (like epoxy, or silicon molds components). Cured processes are generally not simply reversible, being a chemical change.
These terms are sometimes used interchangeably but it's incorrect to do so. Drying is a surface level finish whereby the top coat of your finish is dry to the touch and can usually be overpainted, or walked on etc. It's a much more fragile state than being fully cured.
Curing means that the coating is completely through and through set. It is generally a chemical process and means that the coating is bonded to the surface or the material is in it's full finished state.
Answering from a technical painter's point of view. Let's start with a new concept called the recoat window. For example say that you are going to put two part urethane over two part epoxy (a common application). When epoxy is fully cured It may be difficult to get the urethane to stick (or at least stick as well as it should) so you want to put the urethane on before the epoxy is cured. But on the other hand If you put the urethane on over wet epoxy it will run. Therefore you want to wait until the epoxy is tack free (dry) but not cured before putting the topcoat on. This is the basic idea of the recoat window. (there are exceptions, but the idea is sound)
Evaporative finish = Lacquer, Shellac. The finish will melt/amalgamate into itself. At a later point if touch up is needed this is good. Solids dissolved in a solvent that evaporates when used
Reactive finish = Varnish reacts with oxygen to cure, Epoxy and modern 2 part poly-automobile paints react with a chemical catalyst to cure. Can amalgamate into itself for a window of time. The Epoxies and 2 part paints kick off isocyanates during the curing process which are odorless and particularly toxic.
Coalescing = Waterbased, water is the thinner glycol is the solvent the solids are small droplets of cured reactive finish. Kind of a hybrid of the evaporative and reactive. Can amalgamate into itself for a window of time.
These are all film finishes and not waxes or oil.
Understanding Wood Finishing by Bob Flexner is a great finishing book
Drying is what you get with brine. The water dries, leaving the salt. It's still salt, just not wet now.
Curing is what you get with epoxy. The parts turn into something else entirely.
All paints do both. Curing is what makes them not wash away at the first exposure to their own thinner.
They do not do both at the same rate. That's why you can have very young paint that is dry but vulnerable to physical damage.
2-part paints cure very definitely in a sensible time span (say, 28 days for a marine LPU). With 1-part paints, cure is more of a half-life/log graph stretching out years. I have 50 year old alkyd paints that have cured very completely, and they are as tough as an LPU.
We care about drying for handling reasons. We care about curing for recoat-time, and also for when we can subject the item to the demands of service.
1-part paints are really doing the same thing as 2-part, they are polymerizing their molecules are joining hands to become longer molecules. That is stronger because now a threat must break molecules in two instead of just pushing them apart.
There is a difference between drying and curing.
Drying is essentially the solvent evaporating from the coating. It is the time taken at which the coat can be touched. But other activities like washing can destroy the coating.
Curing is when the coat has reached proper hardness.
For example : oil paint takes 4-8 hours to dry while it takes 7-10 days to cure. Usually curing occurs due to some chemical process like oxidation.
So every coat will have a drying time and a curing time.
Drying = evaporation. Curing = chemical process of molecular change.
You clothes, elmers glue, and paints dry. 2 Part Epoxies Cure. Polyurethanes cure, the second part is water in the form of humidity.
