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I stained this 2 years ago before installation - back and front. The boards were kiln dried and stained inside. The penetrating stain I used contains linseed oil and after I installed I read some reviews saying the stain wasn't very good. I had hoped my cedar being in the soffit would not have any problems but there goes that.

This is the worst one but you can see signs of mold on the others. I tried sanding with 150 grit but the mold seems wet and just smudges.

Any recommendations?

What would you sand with? or would you scrape it? maybe steel wool it? I could go for a dish sponge? I haven't done much work with stain grade wood so any advice appreciated. I currently have scaffolding up so now is the time to fix if there is a time. I don't think I'll spend time on the lesser effected boards unless there is something easy, maybe a topical mildicide? I can't see sanding / staining 140 linear feet at 2' depth being worth while until the lesser effected boards look much worse even then maybe I go with it and stain black.

cedar soffit with mold

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Spray the area with some vinegar and let it sit or 30 to 60 minutes, then wipe off with a cloth or sponge. Vinegar is better than bleach for killing mold because it soaks in below the surface to kill the mold roots. Bleach only takes care of the surface mold. I have used it on my stained deck and it has always worked great and didn't affect the wood color.

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Oh man! Linseed oil, that's food for mold and mildew. Behr was sued for advertising their finish as mildew resistant but didn't include Linseed oil in the ingredient list even though it was well known that linseed oil is food for mildew.

I had a cabin with cedar siding that the Behr finish was applied to. It molded and mildewed FROM WITHIN the finish, not on the surface. External cleaning did nothing. I was included in the class action suit and got enough money to completely strip and re-finish my cabin.

If you are fortunate enough that the mold/mildew you are seeing is on the surface, various topical agents might work, you'll just have to experiment. Don't be hesitant to try several different products, there can be large variations in effectiveness depending upon exactly what you have going on there. I've had good luck with Scotts outdoor cleaner for surface discoloring, brightening, dirt removal.

Worst case scenario, you'll have to apply a stripper and pressure wash off the results. I've had good luck with citrus based strippers, they smell good and are environmentally friendly. Wait until wood is completely dry (several days) before applying a quality finish. Sashco is good, but I'm sure there are others too.

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Find a product that has Oxalic Acid in it. It is usually branded as a wood bleach. It is not Sodium Hypochlorite which is household bleach. It is an oxidizer so wear face, eye, skin, and clothing protection.

I would apply it everywhere as it will take the wood to its "new" form. It will also remove any natural graying from weathering.

Once you complete this, rinse it down and let it dry. Recoat with Benjamin Moore's Arborcoat translucent exterior stains.They are water based, have mold inhibitors included and will protect from rain and sun adequately.

I 100% agree with the poster above about Linseed oil being mold food. Any of the oil-based products tend to mold easily because of this.

Regular cleaning can help inhibit the growth of mold by renewing the mildewcide included in the paint film and removing the dust that mold spores land on and attach to. The dust/dirt keeps the spores from encountering the mildewcide plus provides nooks and crannies for the mold to flourish.

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    Thanks. I'll give that product a try. Unfortunately soffits are 25-30' off the ground so really not sure I'll ever be able to fix the upper ones as the effort required is enormous. I do have some lower down that I'll address at some point. 2 years ago I did vinegar and let dry and then sanded the worst ones and used sansin dec. The worst ones seemed to be the lighter boards - maybe those have less natural mold resistant residues. I used a zero base for natural cedar but still found it quite darkening at least as far as the lighter pieces went. Jul 8, 2022 at 20:07
  • I also did a minwax clear satin waterbased spar urethane on some non-pt plywood that I used outside as a backer to some tight board and batten but it molded as well ! Jul 8, 2022 at 20:08

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