Removing Black stains on Plywood Next to Ribs

[JohnPlatou]

In repairing my 17 Cruisers Inc. 202, I am researching how to remove the blck stains in the plywood next to the ribs on the bottom of the boat.

They appear to be from the nonsealed oak rib in contact with the plywood over the years. The black is under the varnish, so I guess thats has to be removed first, then is there a chemical to remove the black stains.

Are the black stains a sign of other bad problems? Rot I hve not found yet? Or?

The varnish has very few cracks, maybe one every 4 ", along these cracks there is some graying, but nothing like the black in the bottom plywood. The side lapstrake plywood does not show any sign of black.

[Wild Wassa]

There is the staining from moisture (and wood on wood), the metal bits, ... and the staining from oils (like thinned enamels and varnish).

I would try scraping the wood (like a Violin Maker scrapes wood). You don't want to remove more than you need too, I find. You did say that the black was under the varnish. You will soon know if oil, has soaked in.

If oil is in the timber, clean up as best as, ... then use a penetrol type stuff. I would use a cpes (it looks strange in lower case) type stuff. If it is oil from the varnish, the scrapings will tell you, ... sandpaper scrapes just as well, it will gunk-up.

If you find all is dry and non oily, there are plenty of proprietry wood bleaches, use sparingly.

Warren.

ps, One can no longer endorse a product.

[ 10-17-2003, 07:14 AM: Message edited by: Wild Wassa ]

[Bob Smalser]

Sounds like either rust from steel fasteners reacting to the oak's tannic acid or simple blue mold in the plywood veneer's sapwood.

Oxalic acid is probably the best wood bleach out there....crystals dissolved in hot water to a paste...apply with a brush after removing varnish....neutralize with baking soda and water. Any large woodworking store or the "Oxyclean" TV ads.

Will fix blue mold stains easy enuf but iron stains are pretty tough...several applications then learn to live with what's left....stain sometimes goes all the way thru the plywood veneer.

[Mike Vogdes]

Yes John I would eliminate the varnish first as stated above, sometimes just moisture under the compromised varnish will stain the wood an ugly color. Sanding and scraping is your best bet, I find a safety razor blade helpfull in the tight areas, then as Bob describes with the oxalic paste will probably get it right.