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Martin, Electrical Engineer
Category: Home Improvement
Satisfied Customers: 4933
Experience:  Design, construct, fix and grow stuff around and in the home.
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We had a large oak tree cut down, and had about forty 3" and

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We had a large oak tree cut down, and had about forty 3" and 4" sections cut to use in outdoor garden areas as "stepping stones." 1) How and what should I use to "clean" both sides of each piece? 2) How long should I wait until the pieces would be properly dried to apply a sealer? 3) How best to seal both sides of the pieces (and even the bark covering?) and with what type of sealant to preserve the pieces for as long as possible? thanks.
Hello and welcome. That depend of where you are in the USA. That document give a list of EMC Equilibrium moisture content: (page 7 of the pdf, page 4 of the document).
In equilibrium the wood will move the less, in other way it will be more stable.

1) A good way to clean wood is with a pressure water device.

2) Drying wood is an art. You "want" to have it dry fast but that will induce cracking (as the center don't dry as fast as the outer part). A good way is to oil (mineral oil, thin film) both end to slow the drying, causing less stress in the wood. You then wash and sand that oily surface once the drying is completed. To reach the right EMC for your location, you may have to wait a whole year. To measure the moisture content of the wood you have to weight it before and after the drying, the difference will be the water removed (some volatile compound are included but minor compared to water).

3) As for the sealing, polyurethane will degrade with UV and is not very elastic once cured. Linseed oil applied each 2 years would be a good choice. Oak is high in tannins and it give it a good natural fungus protection as it is, the main problem is UV (Ultra Violet) from the sun. So if you can get an oil with UV protection it would last longer.

If the surface eventually become too "rustic", you can always give it a planer pass or a sander pass to "reset" it for another 10 year (you then have to resurface it).

Having a drain and a soil that does not retain humidity would make the slab last longer.

*Note: wood can be slippery, so if you can use fine sand in the finish it would help a lot in rainy day.
Customer: replied 3 years ago.

We live in Florida, thus heat is always a factor almost year-round. In reference to the bark, is it best to treat that as well? The linseed oil is a great recommendation. I had assumed that something like Thompson's Water Seal, which I hate anyway, would not be good.

If you want to keep the bark it will need to be treated as well. You could also give it a spray of copper based fungicide because it may have less tannins than the wood (i never experimented with bark in exterior, only in interior).

In Florida you may require fungicide into the linseed oil because it can cause mildew if the temperature is always hot and humid (it is an organic material after all). Thompson's Water Seal is not that bad of a choice for that application but it does not last as long as most say (as in treatment each year and perhaps more because on a path like that you always walk at the same place).

About the bark, you may want to strip/trim 1/8 to 1/4 of it around the top to that nobody peel it with shoes by walking to close of it. It may tend to peel naturally over time. I would be tempted to remove it and glue it back in epoxy. Again, the only experience i have with bark in the exterior is with firewood cord (i always remove it because the maintenance is easier, but i understand the look and feel it would give).
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