: I have started to revarnish my tern,As I started sanding noticed a dark
: pattern matching size and placement of 3-1/2 in. foam bulkheads,Anyone
: seen this befor?
: I have started to revarnish my tern,As I started sanding noticed a dark
: pattern matching size and placement of 3-1/2 in. foam bulkheads, Anyone
: seen this befor?

Let's think about this. Epoxy is absolutely waterproof, it's just not vapor proof! Your wood is being encapsulated with some moisture in it, say 5-15% by weight. As the relative humidity of the air rises and falls the moisture in the hull's wood will attempt follow it, but there will be a delay as the epoxy sheathing slows the transfer to and from the air. Temperature differences will effect the rate of transfer as will air circulation. For example, if the hull is closed the air temperature will not drop as fast at night inside the hull as outside. The air inside will be able to absorb more moisture initially, but may become saturated. Without circulation the dehumidification will stop. As the air cools the water will either condense on the surface of the closed hull or be re-adsorbed.

Your bulkheads insulate a portion of the inside hull surface from the temperature swings. So this portion will not cool or warm as fast as the rest of the hull around it. Thus the moisture level will be somewhat different. It may be easier for the free moisture in the wood to migrate to this area within the sheathing than to escape. This may account for the discoloration you're seeing. Given a conditioned environment without changes in humidity and temperature the moisture levels in the wood should achieve equilibrium and the discoloration should disappear.

I don't think this effect should result in any long-term effect because we're not dealing with moisture levels high enough to support the growth of fungus. Usually, rot caused by fungus is the result of a breach of the encapsulation that has not been properly sealed. Something like a screw hole. Check out Desperado's project boat at "This Old Boat" if you want to see wood core rot.