Fire Hazards:

Close Calls -
Urethane Scrapings Self-Ignite

In This Article:

Discussion of fire hazard created by urethane material scraped from recently-coated floor.

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By , Editor



This story begins with a mistake made during hardwood floor refinishing. I applied the first coat of urethane around 6:00 pm on a cool Saturday when the humidity was very high, perhaps 95 percent. I knew the urethane would take longer to dry.

On Sunday afternoon I sanded the floor lightly and applied a second coat. By Sunday evening it was evident that something was wrong. The urethane felt rough and had a weird wrinkled surface. Using a paint scraper I found that the second coat would scrape off easily.

On Monday morning I contacted the company and a technical rep told me that I needed to remove the second coat and re-apply it. So I got to work.

Since the urethane had been applied about 18 hours earlier, it was still a little soft and scraping it off was pretty easy.


Within an hour I had scraped the entire 11' x 12' bedroom floor, and made this proud pile of urethane shavings.

This was about 1:00 pm on Monday.

In the late afternoon I began sanding the minor remaining spots of that ill-fated second coat of urethane. A friend came over and I stopped sanding. We chatted and watched the local news.

My friend left and around 6:30, I smelled something odd in the house. It smelled familiar, like almost-dried urethane, yet different... kinda harsh and acrid. I checked the random orbital sander to see if the urethane dust on it had become hot for some reason. Nothing.

I started preparing dinner, but I kept smelling this odor. I noticed it by the fan in the window (which is above the trash can). Was it coming from outside? But the air smelled normal outdoors.

Around 7:00 I thought "is that smell coming from the trash can?". I opened the swinging lid. Yeah, that's the source. It smelled faintly like something was burning, or getting hot. The trash can was nearly empty except for the urethane scrapings and some ordinary kitchen wastes. I touched the lower part of the trash can, and it was quite warm.

I grabbed the trash can and took it outside where it could do less damage if something got crazy.


I grabbed my infrared thermometer and camera. I could smell a story.

I measured the temperature of the surroundings... a cool 69 degrees Fahrenheit.


The lower part of the plastic trash can was 145 degrees.


I took the lid off and looked inside. The mass of urethane scrapings was starting to look brown, and the odor was more powerful.


The temperature of the top surface of this pile was 234 degrees. Something was not right.

I removed the trash bag from the plastic can and set the bag on the driveway.

I spread the bag open to investigate. I was surprised by the burned look of the urethane shavings. The closer to the center

I poked at the shavings to expose the inside. I took this picture of the infrared thermometer reading 360 degrees F, but that wasn't the hottest reading. At one point I measured 425 degrees, but I didn't have the camera ready. Measuring temperature of urethane shavings during spontaneous combustion.


Fire hazard: charred urethane shavings from trash can. With a small stick, I poked at the mass of shavings. It was smoking inside.

While the smoke doesn't show up well in photographs, you can see it (red arrow) in front of the blue trash can.


I used a garden trowel to shovel the urethane scrapings into an empty paint can. The can became quite hot. Safe disposal of urethane shavings or dust: Place in empty paint can.

Then I filled the can with water, and poked at the shavings to make sure the water could reach the bottom

The can immediately became cool to the touch.


But wait... There's more!

All that trouble with urethane shavings happened on Monday. On Wednesday afternoon I applied the real second coat of urethane.


Late Thursday morning I scuff-sanded the second coat with a random-orbital sander. The sander has a porous plastic dust collector cup. It does a decent job of picking up sanding dust.

Normally I would empty the dust collector after sanding for 10 to 20 minutes, perhaps after sanding half the room. But I sanded one entire room and decided to do a better job of cleaning out the dust. I took the sander outside to clean it with compressed air.

But I set the sander down on an outdoor table and got distracted. (Yes I have ADD) Two or three hours later I was outside and I smelled THAT smell again. That sickly acrid burning urethane smell.

This is the dust collector cup after I noticed that smell.

The red arrow points to a melted area on the thick plastic.

(There is duct tape around the cup because I've lost the o-rings that secure the cup to the collector tube.)

Power sander dust collector warped by heat of spontaneous combustion.


Dark colored urethane sanding dust that auto-ignited. There was a thick build-up of dust inside the tube that carries the dust away.

Normally urethane sanding dust will have that whitish appearance, but the dust had started to turn brown from spontaneous combustion.


Inside the dust collector, it looked like a blob of bread dough on the rise.

You can see the brown spots where it's starting to burn.


Smoke from urethane sanding dust that started to burn. I opened up the dust cup and knocked out the dust on top.

Underneath, it was burning pretty good.


I tried to dump out the contents of the cup, but it was stuck. I had to use a long screwdriver to pick out the debris.

The center of the mass of dust was hot and black and smoking.

So I just buried the stuff in the driveway. End of problem.

Charred urethane sanding dust from power sander.

What The Heck Is Going On?

Obviously there is a lesson here... be careful with certain types of sanding dust, they have the ability to self-ignite.

Why do urethane dust or scrapings spontaneously combust? This can only happen if there is an exothermic reaction taking place. Exothermic simply means "giving off heat" (heat exits the material).

When urethane dries, the first thing that happens is the solvent evaporates. I'm no chemist, but all the solvents I'm familiar with "absorb" heat when they evaporate. Common solvents like water, alcohol, mineral spirits, lacquer thinner, acetone (a.k.a. nail polish remover) all feel cool when they evaporate. They require heat to break the loose bonds that keep them liquid, so they cool the surrounding environment. Evaporation of most liquids is an endothermic reaction (heat enters the material), as far as I know. (But I understand there are exceptions.)

I know that after the urethane dries there is still some type of reaction happening, because it takes three days for the urethane to fully cure. You can feel it... the urethane is still soft after it's dry.

I'll make an educated guess that after the mineral spirits evaporates from the urethane, the remaining reaction has nothing to do with evaporating solvents. It must be some other kind of reaction that involves molecules linking together to form a harder substance.

In college I took some plastics technology courses, and we used 2-part urethane to make molded foam parts. This stuff requires vigorous mixing and it foams up and hardens in a couple of minutes. It's similar to the foam in the chair you're sitting in, just not nearly as soft. That type of urethane got really warm, almost hot, within a minute or two.


So Why Doesn't The Floor Start Burning?

While the urethane may be giving off heat after it dries and continues to cure, it's a small amount of heat. If the material is spread out over a large area the heat is dissipated easily. But if the urethane is gathered up (by scraping or sanding) and kept in a pile or a cup, the heat can't escape, so the temperature rises. It's possible that once the temperature rises, the reaction speeds up and gives off even more heat, creating a runaway reaction (or snow-balling effect).


What Else Can Self-Ignite?

I've read warnings about spontaneous combustion on cans of certain Minwax stains. These stains happen to contain linseed oil. I've read that rags soaked in linseed oil can spontaneously ignite. I've never seen it, but I've never used linseed oil. It's an old-fashioned product that was once used for purposes like making outdoor wood more rot-resistant. And it's been used in paints and stains.

Those cans of Minwax stains instructed the user to dispose of rags by placing them inside a sealed can full of water. What a hassle. I usually just burned the rags... but then, I used to live on a big rural spread where nobody minded if I had a little bonfire. The safe thing is: Get the hazardous stuff out of the house. Out of the attached garage too. I would just leave those rags outdoors to dry, weighed down with a couple of rocks.

There might be other materials that can spontaneously combust. It pays to read the instructions, and it pays to take seriously those warnings about spontaneous combustion. I certainly will.


More Info:

Tools Used:

  • Infrared Thermometer
  • Garden Trowel

Materials Used:

  • Empty Metal Paint Can
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Written September 6, 2006