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Most of my bids are painted siding, but I'd like to start taking an occasional larger cabin stain job. It's cedar, and I'd like to know some good practices for treating these black sections. My initial thought is sun damage but what actually causes this? The customer is not interested in the extra costs to media blast back to bare wood OR switching to a solid stain. If this were a deck, I'd just strip, brighten, sand and be done with it, but there are massive sections on the home with this discoloration. I'm assuming pressure washing won't get any of this off. Maybe oxalic acid? Any tips are appreciated, thanks.

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What I “love” about jobs like these are that they let the siding and facia go for years until it gets this bad (unless they just bought or inherited the house) - but they will expect it to look like new when the painter is done.
 

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Like @cocomonkeynuts I'm assuming bad things growing on the wood. Algae. Mold. Whatever. That first pic tells me its probably a shady side of the house. I'd also agree with a bleach wash (or vinegar if you let it soak a lot longer) followed by oxalic acid. Make sure to let it dry out really well before a new stain or whatever goes on it.

And like @RH implied, explain that these things are regular maintenance items. There are no magic bullets. At the very least tell them to wash it down once a year.
 

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Most of my bids are painted siding, but I'd like to start taking an occasional larger cabin stain job. It's cedar, and I'd like to know some good practices for treating these black sections. My initial thought is sun damage but what actually causes this? The customer is not interested in the extra costs to media blast back to bare wood OR switching to a solid stain. If this were a deck, I'd just strip, brighten, sand and be done with it, but there are massive sections on the home with this discoloration. I'm assuming pressure washing won't get any of this off. Maybe oxalic acid? Any tips are appreciated, thanks.

View attachment 114155
We handle a lot of Cedar siding, and we see this dark mildew staining most 1.) when the initial coat is oil based (as mildew feeds off the oils), 2.) when the house is located in the woods where it is shady and/or it is an environment that is damp or humid which promotes these kinds of blooms. They are typically on the North sides, but can be on any side that is in shade frequently, and not ventilated with a breeze, also happens very often on waterfront homes, etc…

Depending on the homeowner, I will inform them of the option of switching to a latex stain, letting them know that semi-solid and solid stains have a longer duty cycle and lower maintenance than other finishes. Often a homeowner is looking for way to limit maintenance, and the idea of re-staining an oil based semi-trans every 3 years is prohibitive (and I no longer handle oil based exterior stains, so I refer them elsewhere in such instances). Latex stains are resistant to mildew, and have a duty cycle on average of 7-10 years in this region. However, it is important to note that it changes the home’s aesthetic- it looks more painted, but retains the texture

Prior to painting, I treat for mildew by doing a soft wash with a pressure washer and running bleach downstream (sometimes adding Jomax).

BEFORE:
Property Building Sky Plant Wood


Window Building Wood Plant Tree


Plant Property Window Building Wood



AFTER (Solid Stain): deep woods, high humidity and shade. Perfect conditions for mildew.
Building Plant Wood House Shade


Plant Wood Building Tree Architecture


another house, similar issue. Moved into Solid stain.Both customers happy with the results

Plant Property Plant community Building Window


Property Building Wood Tree Ladder


The bleach/water will kill the mold spores. In many cases, especially where the staining is severe, the black stains are deep in the wood pores, and difficult to brighten. When switching to solid stain it is not necessary to brighten it completely, only to kill the mildew spore. A mildewcide can be added to stains if the conditions warrant it.
 

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Discussion Starter · #6 ·
We handle a lot of Cedar siding, and we see this dark mildew staining most 1.) when the initial coat is oil based (as mildew feeds off the oils), 2.) when the house is located in the woods where it is shady and/or it is an environment that is damp or humid which promotes these kinds of blooms. They are typically on the North sides, but can be on any side that is in shade frequently, and not ventilated with a breeze, also happens very often on waterfront homes, etc…

Depending on the homeowner, I will inform them of the option of switching to a latex stain, letting them know that semi-solid and solid stains have a longer duty cycle and lower maintenance than other finishes. Often a homeowner is looking for way to limit maintenance, and the idea of re-staining an oil based semi-trans every 3 years is prohibitive (and I no longer handle oil based exterior stains, so I refer them elsewhere in such instances). Latex stains are resistant to mildew, and have a duty cycle on average of 7-10 years in this region. However, it is important to note that it changes the home’s aesthetic- it looks more painted, but retains the texture

Prior to painting, I treat for mildew by doing a soft wash with a pressure washer and running bleach downstream (sometimes adding Jomax).

The bleach/water will kill the mold spores. In many cases, especially where the staining is severe, the black stains are deep in the wood pores, and difficult to brighten. When switching to solid stain it is not necessary to brighten it completely, only to kill the mildew spore. A mildewcide can be added to stains if the conditions warrant it.
I really appreciate the extremely thorough response. Why do you think there isn't any black developing under the soffits? (shown in my picture) That's why I initially thought it was from many years of sun exposure. I live in the Colorado mountains, so essentially high desert. Very dry/low humidity, tons of sun, and not a lot of trees for shade. Doesn't seem like the climate for mildew to propagate perhaps? Again, I could be completely wrong.
 

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Could be sun exposure. OR maybe some type of reaction with the shingles? There is no gutters, so where the black is would be exactly where the sun shines, or where the water run off hits.. Definitely not my area of expertise, but just where the coloring is, and not under the soffits like the OP explained. Maybe test out a wood brightener on it?
 

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I really appreciate the extremely thorough response. Why do you think there isn't any black developing under the soffits? (shown in my picture) That's why I initially thought it was from many years of sun exposure. I live in the Colorado mountains, so essentially high desert. Very dry/low humidity, tons of sun, and not a lot of trees for shade. Doesn't seem like the climate for mildew to propagate perhaps? Again, I could be completely wrong.
Colorado is pretty arid, so mildew seems unlikely, but it sure looks like mildew. It looks like possible snow or rain patterns.

Is it on the North side?

what type of wood is it? In dry climates, left untreated, Cedar turns silvery, not black.

One might have to operate under the assumption that it will be a recuring issue, based on the amount of discoloration and pattern.
 
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