Mystery Dots on my Cutting Boards

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Forum topic by TheWoodenOyster posted 12-07-2013 08:12 PM 1702 views 0 times favorited 23 replies Add to Favorites Watch
View TheWoodenOyster's profile


1313 posts in 1899 days

12-07-2013 08:12 PM

Topic tags/keywords: cherry maple finishing

Hey guys,

I have been making cutting boards for the family for Christmas. Yesterday, I ran into an unpleasant surprise. I sanded three of my boards with 150 and brought them inside. I rinsed the sawdust off of them with water and dried them off with a towel. I left them to dry overnight and came back in the morning. To my chagrin, small greenish-black dots appeared on the end grain of the boards. They mostly appeared on the cherry, but a little on the maple as well (can’t see them on the walnut if there are any). My first instinct is mold, but the boards were wet for a very short amount of time and I didn’t let them just soak. The dots had appeared around 12 hours after I rinsed the cutting boards off. I don’t feel like that is enough time for mold to start growing, especially considering it is freezing in my house. I have attached pictures after one coat of mineral oil/beeswax. Does anyone have any idea what these dots might be? Could they be harmful? Any way around them showing up?

General tips of how you guys finish your cutting boards would be beneficial as well, and may answer the questions above.


-- The Wood Is Your Oyster

23 replies so far

View bobasaurus's profile


3405 posts in 3148 days

#1 posted 12-07-2013 10:06 PM

Could well be mold… I would sand it off and refinish. Try wetting the grain with mineral spirits or alcohol instead of water to remove dust.

-- Allen, Colorado (Instagram @bobasaurus_woodworking)

View Marcus's profile


1163 posts in 1984 days

#2 posted 12-07-2013 11:54 PM

I use a similar process and have never had this happen. If it were me, I would do just what Allen suggested…better safe than sorry.

View CharlesA's profile


3312 posts in 1762 days

#3 posted 12-07-2013 11:57 PM

I’ve never used water to rinse off dust because with some wood, at least, it can raise the grain—mineral spirits, vacuum, tack cloth, microfiber cloth.

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

View JustJoe's profile


1554 posts in 2003 days

#4 posted 12-08-2013 12:02 AM

Did you use purified water to rinse it off, or tap water? If it was tap water and you have any type of iron in it then perhaps iron particulates got down into the pores and then you wiped vinegar on them, causing them to oxidize. That’s just a wild guess.

-- This Ad Space For Sale! Your Ad Here! Reach a targeted audience! Affordable Rates, easy financing! Contact an ad represenative today at JustJoe's Advertising Consortium.

View Biff's profile


126 posts in 1978 days

#5 posted 12-08-2013 01:06 AM

Why are you rinsing the sawdust off with water? If you don’t have compressed air a vacuum or brush would work just as well. A cloth dampened in mineral spirits works, too.

As soon as I’m done sanding mine I blow them off. I dunk mine in a tub full of mineral oil until they won’t take absorb anymore. Then I let them air dry for a day or two for the excess oil to weep out. Then it gets a beeswax rub and it’s good to go!

-- Interested in Oregon property? Visit me at

View realcowtown_eric's profile


608 posts in 1901 days

#6 posted 12-08-2013 01:44 AM

any chance you used steel wool somewhere along the way in the finishing?

or sandpaper contaminated with iron particles?


-- Real_cowtown_eric

View Dusty56's profile


11819 posts in 3652 days

#7 posted 12-08-2013 01:56 AM

Possibly Walnut dust sucked into the pores when you added water to the mix ? Any spots on the sides or faces of the wood, or just on the endgrain ?
What type of sandpaper did you use , and did you wet sand the board for any reason ? I just don’t know why you would rinse dust off the board with water. There are many minerals in tap water that go unseen until they accumulate and dry out.

-- I'm absolutely positive that I couldn't be more uncertain!

View tefinn's profile


1222 posts in 2401 days

#8 posted 12-08-2013 03:34 AM

I’m with the other guys on using water to clean the boards. Why? If you have any mineral content in the water it can stain the wood. Iron turns it black or brown. Calcium, magnesium,sodium and potassium will leave a white residue. It’s best to use mineral spirits if you need to wipe them with anything.

-- Tom Finnigan - Measures? We don't need no stinking measures! - Hmm, maybe thats why my project pieces don't fit.

View a1Jim's profile


117062 posts in 3541 days

#9 posted 12-08-2013 03:49 AM

I think the idea of mold is unlikely unless it had it before you sanded,using steel wool to sand and then using water is a true formula for rust spots, another possibility is that water or the container it was in has contaminates . Is it posible that your wax mineral oil mixture was in a dirty container or to close when your were sanding and some sanding grit got mixed in it.

-- wood crafting & woodworking classes

View TopamaxSurvivor's profile


18246 posts in 3640 days

#10 posted 12-08-2013 04:04 AM

I agree with Jim. I think probably contamination of some kind; walnut dust, iron, or ?? Working with insurance people on water damage sites, they say a week is how long they have before mold becomes a major concern. I doubt mold happened over night.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View lanwater's profile


3111 posts in 2898 days

#11 posted 12-08-2013 04:21 AM

I think it’s most likely the wax trapped in the end grain.
I have seen it on my boards when I stopped at 220 grit. did not see it on board sanded to 400 grit
I did not rinse with anything. just vacuumed between grit.

-- Abbas, Castro Valley, CA

View tefinn's profile


1222 posts in 2401 days

#12 posted 12-08-2013 05:13 AM

I don’t think it was contamination of the oil/wax. If I’m reading the post correctly, the oil/wax was applied after the spots appeared. The spots were most likely from contaminants in the water, or something on the board or in the wood that the water activated.

-- Tom Finnigan - Measures? We don't need no stinking measures! - Hmm, maybe thats why my project pieces don't fit.

View Tennessee's profile


2860 posts in 2479 days

#13 posted 12-08-2013 01:26 PM

Boy, I don’t know for sure, but I’ve seen mold grow on wood in way less than 12 hours if the climate is right. Sometimes when I first cross cut a log and rack it up, I occasionally see mold within 7-10 hours in very small amounts, just like this. The water may have activated the process, as the water that was trapped in my fresh cuts met with the atmosphere. Mold grows fast since it has a pretty short lifespan.
As with the others, why use water at all? I tend to wipe with either lacquer thinner, or at least a dust wipe.
I think maybe sand off the ends, let them dry thoroughly, (as in let the water out), then recoat.

-- Tsunami Guitars and Custom Woodworking, Cleveland, TN

View TheWoodenOyster's profile


1313 posts in 1899 days

#14 posted 12-08-2013 04:55 PM


Thanks for all the replies. Just to clarify, the dots appeared before the oil/wax mixture was applied, so it shouldn’t be contaminates from the oil or wax container. I also did not use any steel wool, it was just sandpaper. The contaminants in the tap water I guess is the most likely candidate. Considering the greenish-black color of the dots, maybe it is copper in the water? Anyway, on to solving the problem.

It sounds like most of you guys vacuum/blow off the dust then use mineral spirits or some sort or quick drying solvent to clean the boards off. I guess I just shy away from that because I am worried about the chemical residue left over from the solvent. I am one of those liability guys who always thinks about how people are going to sue me, so I am a little paranoid about the “foodsafe” materials on cutting boards. I have a shop vac and mineral spirits, so it sounds like I should go with that route over the water route. What do you guys think? Anybody heard anything about chemical residue left on cutting boards?


-- The Wood Is Your Oyster

View Biff's profile


126 posts in 1978 days

#15 posted 12-08-2013 04:58 PM

No problem with residue after wiping with spirits. However, if it’s a concern just use a water-dampened rag.

-- Interested in Oregon property? Visit me at

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