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wrote:
I have a foldable kayak with a interior wood frame. Having left water
in the boat for too long, a number of pieces have what looks like
mildew stain and some minor rotting. The integrity of the pieces are
fine and I am not concerned with the aesthetics. I would like to
refinish the pieces with spar varnish but I am wondering if I need to
get the mildew stains out and how to do that. A light sanding smooths
out the pot marks but doesn't get the stains out. Any suggestions??

There are several ways to kill the mildew, but the most commonly used on
is to pour a bit of antifreeze into the assembled boat and slosh it
around until all surfaces are covered. It will kill the mildew
completely and help prevent it from coming back, but I don't believe it
remove the stains. Doing so probably isn't worth the effort, as the
discoloration is probably too deep into the wood. Any rotted pieces
should be replaced. If you want to coat the frame with something I would
suggest using 100% tung oil from a woodworking supplier (not one of the
"tung oil" finishes from the hardware store). Varnish may look good and
it protects well initially, but when it wears through or cracks, water
seeps in and becomes trapped under the finish, causing rot. A tung oil
finish isn't as waterproof as varnish, but it breathes and allows any
moisture that gets into the wood back out. If you allow the boat to dry
between uses, you won't have any rot problems.

One thing that most people don't realize is that although fresh water
encourages rot, salt water is a good wood preservative. If you paddle on
salt water, slosh some around in the boat and make sure the frame gets
good and wet. Dump out any excess, but DO NOT rinse the boat out with
fresh water, as it defeats the purpose.