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Home Improvement--General/La. Cypress Exterior 10" Post

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Question
We recently build a home with 6 post across the front and back of our house with 8'porches. It was completed in Nov. 2013. We used DeckScapes exterior deck stain in clear from Sherwin Williams.  Now black spots are showing up on the post. What can we do about this? What should we be using to stop this black coloring?

Answer
Dianne,

The black spots you refer to are probably mold/mildew.  You can determine if the spots are under the finish you applied or on the surface by lightly scratching the surface with your finger nail to see if the spots come off easily.  If they do, that would indicate the spots are on top of the finish, if not, it would indicate the spots are under the finish.

Cypress has a high moisture content and if not dried before use mold can develop.  Depending which stain (water based or oil/alkyd based) was used the mold could develop on the finish or under the finish.  Removing it and preventing it from returning requires a different approach depending on which stain was used.  If the mold is on the surface a mixture of 3-4 parts water and one part bleach with a soft bristle brushing will remove it, but bleach will not kill mold.  You need to apply a microbial/mildewcide after bleaching.  If the mold is under the finish you need to remove the finish then bleach and apply the microbial/mildewcide, then re-apply the finish.

Sherwin Williams makes several deck products, and to my knowledge they all contain a mildewcide additive, but each has particular properties and each present different issues when trying to correct any given problem.

Before you do anything I would confirm that the spots are mold.  It is possible they are not, but are extractions of tannins in the wood caused by reaction with chemicals in the stain formula, particularly certain solvents.  I have seen this effect in cedar and redwood on several occasions.  I would recommend you go to a Sherwin William store and discuss the problem with the manager or knowledgeable representative of the store and get their input and recommendations.  I hope this was helpful.  Good luck.  

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Len Kroll

Expertise

I can provide advice on the proper design, construction, maintenance, restoration, preservation, cleaning and waterproofing of log homes. Questions concerning proper cleaning, sealing, staining and waterproofing are within my area of expertise. My expertise includes new construction and existing log structures. Chinking application or restoration, log replacement and repair are also in my area of expertise.

Experience

40 years experience in the log home and construction industries, 10 years in specialty contracting, 30 years in the log home field. We have designed, produced, supplied and built over 3,000 log homes and restored/repaired over 600 log homes. See more information at our web site: www.woodhouseloghomes.com.

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Authored a monthly column expressing the Vice President and General Manager's "Observations and Insights" distributed to 300 employees and management.

Education/Credentials
BS degree in accounting, University of Illinois, 1963. Master of Business Administration degree, University of Chicago Graduate School of Business. Numerous relevant seminars including an intense two week business administration seminar, University of Pennsylvania Wharton School of Business, 1978. National Home Builders Association Graduate Builders education program.

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National Honor Society Student, Illinois State Academic Scholarship, Dean's List.

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