Speed Up Finish Drying with Heat

[Stan Suther]

I've been turning bowls for about a year, and have experimented with several finishes. Coming from making furniture, I'm used to laying down a finish and waiting overnight before adding another. With bowls, I find I have less patience. I've been enjoying using General Finishes Arm-R-Seal. It's easy to wipe on and provides a nice sheen. However, it suggests waiting 12 or more hours before adding another coat (it's quite thin). Just wondering if anyone has had success with speeding up the drying/curing process on small items using heat from a heat gun or hair dryer. Alternately, how about using a microwave oven to speed things up? Thanks in advance for your answers.

[Paul Williams]

I have not used Arm-R-Seal, but several times I have had oil finishes that were not setting as quickly as I needed and used the oven. I preheated the oven to 120 degrees, turned it off and put the bowl inside. Let it cool for 30 minutes or so and repeated 3 or 4 times until dry. I quit. Not because it didn't work, but the last one I left in the oven to cool and forgot. My wife decided to make a pizza. After preheating the oven to 425 set the pizza in the oven pushing the small bowl to the back. 30 minutes at 425 resulted in a very rich darker finish. I consider it lucky it didn't result in a fire.

[Dave Mount]

Have not tried it, but I don't think the microwave is a good idea. Microwaves work by exciting water molecules, of which there aren't any in the finish, but there are plenty down in the wood. Driving water vapor (steam) up out of the wood and through your newly applied finish is a recipe for disaster. Again, working from theory not experience, but I'd be willing to put a fair wager on it.

I've tried hot air on slow drying finish, not on bowls, but on other wood. My experience is that what happens is that the finish dries on the surface (skins up) but there's still lots of solvent down deep that needs to be released, and now you have cured finish covering uncured finish and you get wrinkling and cracking.

If you want to speed finish curing with heat, I'd suggest making a small insulated box with an incandescent lightbulb in it. You want to gently raise the temperature of everything, not strong spot heating. Good idea to put your work in there before applying finish too. If you apply finish over a cool piece and then raise the temperature, gas expansion down in the wood can push bubbles out through the finish. Common problem with epoxy, wherein the finish is too thick/viscous to allow the bubbles to escape and you get them embedded in the epoxy layer. Might not be a problem for a thin finish like ArmRSeal, but why take the chance.

If you want to go fast, shellac or lacquer might be better options than heating polyurethane.

Best,

Dave

[Richard Coers]

two things can go wrong. If your bowl still has a little moisture in it, you will get cracking from the sudden heat shock. Secondly, there is a difference between dry to the touch and cured. If you dry it quickly, you can get a hard surface and solvent trapped under it. That will be about some issues when you put on the second coat. I never second guess the scientists that set the rules for the products they developed.

[Kyle Iwamoto]

If you dry it quickly, you can get a hard surface and solvent trapped under it. That will be about some issues when you put on the second coat.

+1.... I've put a coat over the first coat that was not completely "dry". Result? That first coat will never dry after you top coat it. It will cause you problem after problem until you sand off the layer, let it dry, and start over..... Rushing causes waiting. Haste makes waste. Patience is a virtue.

[Pat Scott]

I used to use Arm-R-Seal (which is basically Danish oil), and have learned not to rush the 2nd coat. Like others have said, if the 1st coat isn't fully cured (not just dry to the touch) before putting on a 2nd coat, the 2nd coat takes days to a week to cure. I now let the 1st coat cure for 3 days before putting on another coat. It's faster to wait 3 days for the first coat to cure than it is to rush a 2nd coat.

[John Keeton]

Early on in the turning game we all have wanted to rush a finish. After all, for those of us that did flat work for years the idea of completing a project in a day or two made us feel the finish should go quickly, as well. But, as others have said, unless you are using lacquer or other “quick” finishes, the reality is that the oil/polymer based finishes have to polymerize, i.e., cure. That process is a chemical transition and takes time. IMO the addition of heat is a hindrance to this process.

I have always viewed “finishing the wood” and “finishing the finish” as two different processes. One problem that can occur with a rushed finish is the inability to work with/finish the finish. I typically will go thru a series of built up layers, then sand to level the finish, add more coats, and sand again working toward a nice, level finish that can be buffed to whatever sheen you might want - free of nubs, unleveled areas, runs, etc. In order to do this you MUST have a hard, cured finish.

[Brice Rogers]

I made a food dehydrator - - just a wooden box with a pair of light bulbs, some racks, a computer (exhaust) fan and an automobile air filter. It is about 24" x 24" x 18". Worked great with things like apple slices, jerky, peach slices, etc.. With one bulb, I think that the temperature is around 100F. Of course, that depends on the wattage of the bulb. I seem to have the best bulb life by buying the incandescent bulbs at the dollar store (surprising!).

Then I started using it for drying Min-wax (oil-based) poly. The intent wasn't so much that I wanted to really speed things up, but more that I wanted to make sure that the poly cured between coats. So, I typically coat the piece, and then move it into the "oven". let it run over night and re-coat it the following day.

[John Keeton]

Brice, in San Diego, I would think 100* in your dehydrator would require an AC unit!!:D It has been in the mid 90s here for too long.

I don’t see a problem with that kind of situation, but that is far different than using a hair dryer or heat gun.

[Mike Nathal]

There is also a technique where you apply the DO or poly, wait a few minutes and then wipe off all the excess. Sometimes it is even described as "scrubbing" the excess off. It leaves an ultra thin layer remaining. It requires a lot of coats to achieve a high gloss finish, but the plus side is that it is ready for the next coat in as early as an hour. Another advantage is the little specks of dust stuck in the coating are reduced to almost nothing. I don't follow this practice rigorously but I do wipe the excess off more than I used to, and it does cure faster, and with fewer dust specks. .

[Stan Suther]

When using ArmRSeal, I follow the directions and wipe off the excess. It leaves a very thin coat which feels dry to the touch within an hour. They still recommend the overnight wait for curing. Since it is so thin after that short dry time, I'm tempted to add several coats in a day. I'll try that on a sample and see what happens. I've also used wiping Poly which has pretty much the same instructions. For bowls, I don't like to spray a finish.

[Dave Mount]

ArmRSeal *is* wiping poly -- it's urethane varnish per the manufacturer.

Someone else said ArmRSeal is the same thing as Danish oil. That isn't true, Danish oil is an oil/varnish blend. ArmRSeal builds faster and dries harder. Not saying one is better than the other, just saying they aren't the same thing, despite having similar application instructions. Lot of turners like Danish oil because it has a more "in the wood" appearance rather than looking like a coating over the wood. Again, all depends on what you're after.

[Thomas Canfield]

I did not see any statement about raising the temperature will also lower the humidity that helps with the curing process. I made a drying box out of a shop vac box with vent holes top and bottom adding a rack for wire shelves (section of closet wire shelving sitting on dowel rods) and a brooder lamp using only a 40 or 60 watt incandescent bulb. The temperature rise is usually about 20 to 30F. Usually leave in for 24 hours but longer when high humidity or cold temp below 50F. Makes a big difference in cure time and quality of drying.

[David Walser]

Dave -- I'm not trying to be pedantic, just clarify the terms. The term 'varnish' refers to a broad category of film finishes, not to a specific chemical type of finish. That is, lacquer, shellac, and polyurethane are all types of 'varnish'. Here's what Wikipedia has to say on the subject: "The term 'varnish' refers to the finished appearance of the product. It is not a term for any single or specific chemical composition or formula. There are many different compositions that achieve a varnish effect when applied. A distinction between spirit-drying (and generally removable) 'lacquers' and chemical-cure 'varnishes' (generally thermosets containing 'drying' oils) is common, but varnish is a broad term historically and the distinction is not strict." (I quote Wikipedia because we all know there's nothing but truth on the internet.)

Thus, referring to Danish oil as an oil/varnish blend is not inaccurate. That's what it is. Of course, the description, while accurate, isn't all that useful. Each manufacturer has its own formula for Danish oil and there is a wide variety of combinations of oils and varnishes that are used in differing proportions. So, Watco Danish Oil might build far less quickly than another brand, but that doesn't make one a true Danish oil and the other something else. Again, quoting Wikipedia:


"Danish oil is a wood finishing oil, often made of tung oil or polymerized linseed oil, although there is no defined formulation so its composition varies among manufacturers.

Danish oil is a hard drying oil, meaning it can polymerize into a solid form when it reacts with oxygen in the atmosphere. It can provide a hard-wearing, often water-resistant satin finish, or serve as a primer on bare wood before applying paint or varnish. It is a "long oil" finish, a mixture of oil and varnish, typically around one-third varnish and the rest oil." [Emphasis added.]

I am NOT saying ArmRSeal is or is not a Danish oil. I've not looked at its formulation. But, to the extent it is a blend of oil and a film finish, it's a Danish oil. It's also a varnish.

[Dave Mount]

Dave -- I'm not trying to be pedantic, just clarify the terms. The term 'varnish' refers to a broad category of film finishes, not to a specific chemical type of finish. That is, lacquer, shellac, and polyurethane are all types of 'varnish'. Here's what Wikipedia has to say on the subject: "The term 'varnish' refers to the finished appearance of the product. It is not a term for any single or specific chemical composition or formula. There are many different compositions that achieve a varnish effect when applied. A distinction between spirit-drying (and generally removable) 'lacquers' and chemical-cure 'varnishes' (generally thermosets containing 'drying' oils) is common, but varnish is a broad term historically and the distinction is not strict." (I quote Wikipedia because we all know there's nothing but truth on the internet.)

Thus, referring to Danish oil as an oil/varnish blend is not inaccurate. That's what it is. Of course, the description, while accurate, isn't all that useful. Each manufacturer has its own formula for Danish oil and there is a wide variety of combinations of oils and varnishes that are used in differing proportions. So, Watco Danish Oil might build far less quickly than another brand, but that doesn't make one a true Danish oil and the other something else. Again, quoting Wikipedia:
"Danish oil is a wood finishing oil, often made of tung oil or polymerized linseed oil, although there is no defined formulation so its composition varies among manufacturers.

Danish oil is a hard drying oil, meaning it can polymerize into a solid form when it reacts with oxygen in the atmosphere. It can provide a hard-wearing, often water-resistant satin finish, or serve as a primer on bare wood before applying paint or varnish. It is a "long oil" finish, a mixture of oil and varnish, typically around one-third varnish and the rest oil." [Emphasis added.]

I am NOT saying ArmRSeal is or is not a Danish oil. I've not looked at its formulation. But, to the extent it is a blend of oil and a film finish, it's a Danish oil. It's also a varnish.

The products marketed as "Danish oil" (e.g., Watco and Deft) are dramatically different finishes than ArmRSeal. I take it the issue is whether calling Danish oil an "oil-varnish blend" creates a useful distinction.

The source of my understanding is Bob Flexner's book, "Understanding Wood Finishing." I don't know if that trumps Wikipedia, but it provides a different perspective than the one in your post. To the best of my understanding, what I say below is consistent with the information in that book.

Since no manufacturer provides us with the recipe for their products, and they exist along a continuum without defined boundaries, terminology is tough. Flexner makes a distinction between "oil/varnish blends" where he places both Deft and Watco "Danish oil", and "varnishes" where polyurethane resides. And from what the manufacturer says about it, and my own experience using it, ArmRSeal is in the category of products commonly marketed as "oil-based polyurethane".

Flexner says, "Varnish is made by cooking one or more oils with natural or synthetic resins. The heat causes the oil and resin to combine chemically, forming an entirely new substance." Though he doesn't say it in this way, this implies that what he calls "oil/varnish blends" are a combination of varnish (which has oil as an ingredient, but is no longer in its original form) and additional oil that is added to the varnish. Elsewhere, he says that the characteristics of varnish are influenced by the ratio of oil to resin. The text isn't explicit on this, but it implies he is talking about the ratio of oil and resin that are heated together, not that it is regular oil added to varnish, which seems to be what he terms an oil/varnish mixture. He says spar varnish is flexible because it has a higher oil to resin ratio, but he calls it a varnish, not an oil/varnish blend, and therefore not in the class of finishes he puts "Danish oil" in.

I found a blog post by Flexner that is consistent with the above viewpoint, though it also is not explicit about it. https://www.popularwoodworking.com/flexner-on-finishing-woodworking-blogs/the-secret-finish-formula-that-isnt-so-secret/

Flexner offers a procedurally defined diagnostic test for oil/varnish blends where you put a drop on a piece of glass and let it dry overnight. What Flexner terms "oil/varnish blends" (which is the category he puts Deft and Watco Danish oils in) dry wrinkly and soft. What he classifies as a "varnish" dries smooth and hard, which ArmRSeal does.

Flexner doesn't offer a compositional threshold for when something ceases to be a varnish and becomes an oil/varnish blend. One can argue that 0.01% varnish in BLO is an oil/varnish blend as is 99.99% varnish with 0.01% BLO, but I'm not sure I see the value in it. There exists a continuum in automobile design between cars and SUVs that might make it hard to say exactly where one becomes the other, but that doesn't mean that "car" and "SUV" aren't useful terms. The inference from Flexner's book is that he would call it an oil/varnish blend when it has enough of each to create a finish that has characteristics that are perceivable as being some combination of both types of finish.

To the extent one accepts Flexner as an authority, I think the sum of all that supports what I said. If it doesn't, it's probably better that you read Flexner for yourself and see if I misunderstood it, because I'm just regurgitating what I learned (or thought I learned) from the book. Maybe Flexner will stop by and respond himself, he used to drop by WoodCentral occasionally.

Best,

Dave

[Jay Michaels]

I've used Arm-R-Seal as my go-to finish on most projects for years and the one thing I've found that consistently dictates drying time is the humidity level in the room. That has a much greater effect on drying time than just fluctuating temperatures (not counting oven-drying in this - just air drying).

When I apply that first coat to a project in the winter, I can easily apply a second coat (and sometimes even a third) on the same day. But in the summer, I routinely have to wait a full 24 hours (or more) before I can proceed to the next coat.

I've resorted to the oven on occasion - and the heat helps, but it is primarily the fact that the oven creates a particularly DRY (low relative humidity) environment that aids curing, in my opinion.

[David Walser]

Stan -- You've received several good responses to your original question -- whether it's a good idea to use heat to speed up Arm-R-Seal's curing. Allow me to address a question you didn't ask: Is Arm-R-Seal the 'right' finish for the bowls you're turning? Generally, I prefer to use film finishes, such as lacquer, shellac, and polyurethane, on decorative pieces. For utility items, I prefer a penetrating oil finish, such as BLO or walnut oil. Utility items get handled a lot. My experience that items with a film finish quickly look 'dingy' when handled a lot, while the same item finished with oil just develops a 'nice patina'. And, when a utility item needs it's finish refreshed, that's generally easier with an oil finish than with a film finish.

For these reasons, it's been a long time since I used a film finish on a bowl. My general practice is to wet sand the bowl with walnut oil. By the time I've sanded through the grits, the bowl has had its finish applied. You really can't get much quicker than that.