Mildew on Exterior Plywood

[dlu]

We live in the Pacific NW (USA) and on exterior plywood surfaces painted with Benjamin Moore exterior latex (supposedly with a mildew inhibitor mixed in) we're seeing a black growth that I assume is mildew. This only seems to be happening on plywood surfaces. In the photo below you can see it on areas of the door, but not around the edges. The door is a plywood lamination with solid wood "edgebanding" -- notice how the mildew does not reach out the edges of the door.

Any thoughts about why this might be happening? It's happening in a number of locations, with different plywood (mostly Arauco ACX underlayment and a sande marine plywood) and different -- I think -- paint batches.

 

[peter halle]

Is the mildew actually on the surface or in the finish?

Peter

 

[squall_line]

Do you have any closeups?  As Peter said, is it on or in the finish?

I'm wondering if it's actually a growth or if it's some of the pigment settling out of the paint.

 

[woodferret]

Arauco ACX is pine.  The glue is outdoor rated... the wood... is pine.

 

[dlu]

Is the mildew actually on the surface or in the finish?

It looks like it's mostly on the surface, but there is also some that seems embedded in the paint. In the photo below you can see a small area that I cleaned with bleach. A lot of the black came off, but there are still some black dots in the area I cleaned.

 

[peter halle]

I am going to venture a guess that the mildew is being caused by moisture coming thru the paint from the wood.  If you look at the images you have numerous horizontal seams / cuts that could be allowing moisture in.  The mildew looks to be more pronounced directly under those - i.e.  the cutouts for the windows.  Of course latex paint also breathes so in effect it isn't waterproof but rather water repellant so that could also help increase the moisture content overall but compounding those areas.  It would be interesting to see what a moisture meter would read in the different areas.

MY guess.

Peter

 

[usernumber1]

to me this looks like condensation forming and the areas where there's no mold is where it's drying out. there's no paint in the world going to help you there in the PNW where you have dampness that doesn't dry out or sees the sun

 

[Packard]

Is the door protected from the rain?

Does the door have any sun exposure?

What does that door lead to?  Is it a heated room?

I’m thinking perhaps condensation might be the culprit.  If the room is not heated, then the plywood would have less insulation and might get more condensation.

You can check on that.  Several times a day over several days, check to see which surfaces feel damp or wet and which feel dry.

If it is a garage, then attaching a sheet of foam insulation to the interior might be the remedy.  In a garage that might be acceptable.  In a house interior, that would not.

The Pacific North West is known for its abundant rainfall and I would presume high humidity days.

If the door has direct exposure to the rain, then awning of some sort might be the answer.

Sun light, UV exposure, will prevent the formation of mold and will kill mold that has already formed.  But adding UV exposure is difficult (who wants a front surface mirror as landscape ornament?).

More information might be helpful.

All just guesses.

Addendum:  I see User#1 covered some of the same points while I was typing.

 

[dlu]

Is the door protected from the rain?

Somewhat. It's on the lee side of the building, and has a bit of an awning over it. The deck just outside of the door is dry at the threshold even in "typical" rain.

Does the door have any sun exposure?

In theory yes, but we can go weeks without sun exposure this time of year  [smile]

What does that door lead to?  Is it a heated room?

Not usually. It's a storage/project shed and not getting much use this time of year. The mildew started in the summer. The shed was heated in the fall. I'm not sure if it's gotten worse.

I’m thinking perhaps condensation might be the culprit.  If the room is not heated, then the plywood would have less insulation and might get more condensation.

You can check on that.  Several times a day over several days, check to see which surfaces feel damp or wet and which feel dry.

If it is a garage, then attaching a sheet of foam insulation to the interior might be the remedy.  In a garage that might be acceptable.  In a house interior, that would not.

The Pacific North West is known for its abundant rainfall and I would presume high humidity days.

It could be condensation, but it seems a bit odd that with the whole shed at ambient temperature that the walls aren't getting an equal amount of condensation. The only places that are showing mildew are vertical plywood surfaces: nothing on the Hardie Plank walls, or on the solid wood trim, or even on the horizontal eaves. Just on the doors and some plywood sheafing in the peaks above the planking.

A Benjamin Moore rep came out to look at the mildew yesterday, he says that it's common around here and that the only solution that he knows in repeated cleaning with a product (bleach or vinegar) that will kill the mold spores. He says to expect it to get better after the first cleaning, but that it will take several cleanings to completely eliminate the problem, and he comped us a bottle of the store brand cleaner. Gotta love dealing with local vendors.

 

[Packard]

Consider using hydrogen peroxide to kill the mold.  The stuff you buy in the drug store is 3%, Home Depot carries 12%.

The chemical is fairly benign.  In the 3% it is used for wounds and as a mouth wash.  I would not use 12% for those purposes, but it is less dangerous to use than most.

On the downside, it takes several applications to kill the mold.  (Killed mold is much easier to wash off than live mold.)

 

[woodferret]

I'd be more inclined to use Concrobium Mold Control.

 

[luvmytoolz]

A Benjamin Moore rep came out to look at the mildew yesterday, he says that it's common around here and that the only solution that he knows in repeated cleaning with a product (bleach or vinegar) that will kill the mold spores.

After having to deal with a really bad mold problem in a rental many years back, we tried bleach and vinegar on the mold and it did nothing, later after researching it I was of the understanding that bleach and vinegar are only good for cleaning mold off non-porous surfaces, but don't actually kill the mold itself. And for timber/fabric/plaster walls, etc, it's useless as it doesn't penetrate to the core of the mold.

 

[woodferret]

Concrobium is mostly just basic salts.  Oddly, mold tends to spore more in low pH conditions so 'killing' with vinegar seems counterintuitive to me.  Sure, pH2 will kill things, as will the bleach, but that action is short lived and the residuals (at least the vinegar) when diluted makes for nice breeding ground.

The basic side of things tends to retard the spore attachment.  When the concrobium is dried, it basically reverts back to sodium carbonate, trisodium phosphate, and sodium bicarbonate crystals.  When they get 'wet', that alkaline film pretty much says go away.  It also doesn't flash off like bleach.

edit: I guess the downside is that it'll look like junk.  The stain will still be there, and as the concrobium dries, it'll leave a whitish salt.  Might still be worthwhile to get it in behind the siding where there should have been an airgap, or pores of the plywood.

 

[Packard]

If we go on the assumption that the plywood is singled out for mold, then condensation is the likely cause. 

I would try screwing a piece of expanded foam wall insulation to the back side of the door.  We have established that this is a storage shed, so appearance on the interior is not likely to be a factor. 

The foam panels are so light that no change in the structure of the door or the types of hinges will be required.

I would note that once the mold is killed, it will wash off fairly easily.  If after the first application of the bleach, hydrogen peroxide or other mold killer, the mold does not all wash off easily, then apply additional applications.

Hydrogen peroxide is recommended for non-porous surfaces also.  It will kill mold.  The 12% solution (from Home Depot) will work faster than the 3% stuff from the drug store.

My preference is because the Hydrogen peroxide is softer to use than bleach, especially outdoors where the wind can blow it back on you.

Read the label.  It is likely that eye protection and long sleeve shirts are necessary.

 

[dlu]

Thanks all! I think we'll start with a good cleaning, HO sounds like a good way to start. I'm wondering about the "nonporous" surface issue, seems like with several coats of paint the plywood is "kind of nonporous" so surface treatments might work. Maybe get it cleaned and then apply Concrobium?

 

[Laminator]

I have used a product from Lowe's called "wet & forget"
  It takes a period of time (a few weeks) from application till you see the results but it lasts for a year or two.  I've used it on concrete patio and plastic patio furniture and it makes it look brand new.  I live in East Texas which includes high humidity that mold and mildew thrive on.