Oxalic Acid

Just hearing the word "acid" sounds ominous. But oxalic acid is easy to use and the safest for the home. In fact it is found in many vegetables including spinach, and rhubarb.
Perhaps its best known use is that as a wood bleaching agent.
Exterior wood tends to turn a dark gray after being exposed and Oxalic acid solutions are used to lighten and brighten dingy gray wood.
In the refinishing of wood furniture, oxalic acid can be used after stripping to lighten and soft darker stained areas befor refinishing.
a few other uses for oxalic acid are:

• Precipitating agent in Rare-earth mineral processing;
• Bleaching agent in the textile activities, wood pulp bleaching;
• Rust remover for Metal treatment;
• Used in commercial rust removers to remove rust stains from tubs and sinks.
• Grinding agent, such as marble polishing
• Waste water tratment, removing calcium in water.
• Use in cleaning and sterilizing homebrewing equipment.
• Useful as a reducing agent for photography and ink removal
• Used as purifying agent in pharmaceutical industry
• Rock collectors,clean mineral specimens
• Remove food and rust stains from kitchen countertops, plumbing fixtures and fabric

Wood Decks:
After scrubbing with Oxygen bleach and TSP ( trisodium phosphate ) use Oxalic acid to lighten the wood prior to sealing or staining.

Use to Prepare Furniture for Refinishing:
A detailed and comprehensive article from the Department of Environmental Design, University of Missouri-Columbia.

Cleaning Stains and Deposits on Household Surfaces:
University of North Carolina Cooperative Extension

How to clean mineral specimens, written for rock collectors.

Other uses I have found:
Mild solutions will remove rust and ink from fabric
Hard water stains are easily removed from exterior surfaces like driveways, garage floors,stucco and painted surfaces.
Great for cleaning hard water stains from plumbing fixtures, ceramic tile and shower enclosures.
Makes Lawn and patio furniture look like new.

Where Can I get Some?

Warning: Oxalic Acid is still an acid and should be treated with respect, do not be lulled into complacency.
In its purified form, oxalic acid is a highly corrosive and toxic substance.
Latex gloves should be worn at all times when using it and it should always be mixed and used in a well-ventilated area.
The gasses given off are dangerous and have been suspected by some in causing health problems.
Always protect your skin,wear a dust mask when mixing ANY powder, eye protection is a must and whenever I am fooling around with chemicals I always have a supply of fresh water available for flushing off spills. If swallowed, give plenty of milk or water, preferably water. Do not induce vomiting. Contact a poison control hotline. If inhaled take a break and get some fresh air. If the nausea or headache persists, contact the poison control hotline. Used Oxalic Acid should be considered hazardous waste and should be disposed of properly.