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Making sawdust in MS
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Should I sealcoat a tabletop on bottomside, or does it matter? It's cypress, 42x84 inches. I've heard it both ways. And if so how many coats.
 

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How many coats depends on the finish. I would put enough finish on the underneath side that it looks like it has a coating. It's just to seal the moisture out so the humidity doesn't make the underneath side swell and maybe warp.
 

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Before I came to this forum I had never heard anyone advocate not sealing the bottom. To me its common sense. A board warps when the moisture content on one side goes higher on one side than the other. If you seal only one side it invites the moisture content on the raw side to raise higher than the other.
 

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Before I came to this forum I had never heard anyone advocate not sealing the bottom. To me its common sense. A board warps when the moisture content on one side goes higher on one side than the other. If you seal only one side it invites the moisture content on the raw side to raise higher than the other.
Well stated and as for those who say different,here's how to find out without sacraficing a table top.
Lay a long board on the ground and each end will rise skyward as the underside absorbs more moisture than topside.
Flip the board over and it will curl from the opposite side. For our purpouses,cupping from edge to edge that occurs for the same reason is often a larger concurn
Some species reacte more than others but unless there is valid reason to leave one side of a panel unfinished,cedar storage chest for instance,it's good to seal the back.
 

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Rick Mosher
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I find the usual reason not to seal the bottom is laziness or trying to save some money.

Moisture content is probably the biggest reason but balancing the mil thickness is another.

If I spray a fully filled coat of polyester on a top and don't balance with an equal build on the bottom I will have warping issues as well, even though there is no moisture getting through the unbalanced finish.

Just like in veneering you must balance a panel by veneering both sides and they must be similar thickness and species.
 
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