18

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?

  • I think you may be talking apples and oranges here. Finish v. Oil. The finish sits on top of the wood with the distillates (water or petroleum) evaporating out to allow it to finish up. The Tung oil absorbs into the wood to form the finish with no distillates to evaporate. I'm not an expert, so leaving this as a comment. – Pᴀᴜʟsᴛᴇʀ2 Apr 10 '15 at 12:15
  • 2
    This is a good question to receive a definitive expert answer, though I'm worried that it's going to yet be another, "Each person and manufacturer uses these terms differently, so there's no real standard." – Adam Davis Apr 10 '15 at 15:40
  • @AdamDavis there certainly seem to be a lot of those issue where wood working in concerned. That's why we let the community decide. Feel free to bring things like that up in Meta. There was a meta post about this exact topic already I think. I know it was brought up in the WorkShop – Matt Apr 10 '15 at 18:48
  • Can I put wooden shelves before the cure time is up? – Suzanne Oct 9 at 7:41
  • Hi Suzanne, StackExchange is not a forum where you can tack on another query like this. SE is a question-and-answer venue and you've posted where only an Answer should go, see the Tour. Please use the search feature, it works extremely well here, and only then ask your own Question if you can't find an existing Q&A the covers this (you will by the way). – Graphus Oct 9 at 13:32
15

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.

13

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.

5

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.

  • Do you have resources that back this up? Technically ewm's answer is correct from a chemical/physical standpoint, but without references to expert literature this answer just appears to be opinion. Is this how professional woodworkers define these terms? – Adam Davis Apr 10 '15 at 15:39
  • Would another distinction be that drying requires exposure to air and curing does not, such that one must wait for a finish to dry before applying more, but no such delay is necessary for curing? – supercat Apr 10 '15 at 15:52
3

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)

2

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

1

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.

  • Do you have resources that back this up? Technically ewm's answer is correct from a chemical/physical standpoint, but without references to expert literature this answer just appears to be opinion. Is this how professional woodworkers define these terms? – Adam Davis Apr 10 '15 at 15:39
  • Yeah i said this from my own readings before..But EWMs answer with wikipedia links should clear things i suppose..Thanks for pointing out .. ill add links to sources next time :) – Imprfectluck Apr 13 '15 at 19:18
1

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.

  • 1
    Paints cure too. Behr even lists a 4 week cure time on their website. – Doresoom Jul 24 '15 at 21:45

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