Repair a Water-Damaged Finish - Popular Woodworking Magazine

Repair a Water-Damaged Finish

 In Questions And Answers, Techniques, Wood Finishing

Repair a Water-Damaged Finish

Work miracles on wood with oxalic acid.

By Kevin Southwick

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Watering a potted plant can be disastrous if the plant
lives on top of something made
out of wood. We’ve all seen the
white spots and black rings that
can result when water seeps
through the pot. And if you’ve
ever tried to sand out these marks,
you know it’s a tough job that can
leave telltale depressions on the
surface. Fortunately, in many cases,
this type of damage can be almost
magically undone by treating the
wood’s surface with oxalic acid.

Oxalic acid removes the gray
color from oxidized wood, without
changing the wood’s natural color.
That’s why it’s commonly used
as the active ingredient in deck
cleaners, and why restorers use
it to remove gray or black water
stains on furniture (see “Oxalic Acid
Undoes Rust,” below, right). Oxalic
acid is also used in some household
cleaning products for removing
hard water stains, and it has many
industrial uses as well. Although
it is found as a natural ingredient
in some vegetables (spinach and
rhubarb), oxalic acid is quite toxic if
ingested in concentrated form.

Oxalic Acid Undoes Rust

Oxalic acid works a specific type of chemical magic by
removing dark water stains from wood. These stains result
when water containing iron and other minerals gets into
wood. The discoloration that occurs is similar to rust. As
shown here, oxalic acid is strong enough to dissolve the rust
from an old plane iron, but it has little or no effect on the
non-oxidized steel. (This plane iron soaked overnight in a
saturated solution of oxalic acid.)

Oxalic acid is uniquely different from the other two
bleaches occasionally used in wood finishing and refinishing,
because neither chlorine bleach nor two-part bleach will significantly
affect water stains or rust. Chlorine bleach is good
for removing or lightening dyes (as it does in the laundry).
Two-part bleach will lighten the color of just about anything
it can soak into. Part A is sodium hydroxide (lye); Part B is
hydrogen peroxide (which is also used to lighten hair color).

This story originally appeared in American Woodworker October/November 2009, issue #144.

October/November 2009, issue #144

Purchase this back issue.

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