BARREL PREPARATION TREATMENT
The following procedures are used in commercial wineries to remove excessive oak tannins and to sanitize barrels:
1. To five (5) gallons of hot tap water add one (1) pound of soda ash and two (2) teaspoons of household lye. Make enough to fill the barrel. When you are sure that the ingredients are completely dissolved, pour the solution into the barrel.
2. Let the barrel soak for two (2) to three (3) days.
3. Drain the solution and flush the barrel with clean water three (3) times.
If you are preparing an all-new, unused oak barrel, repeat steps #1 through steps #3 at least two (2) times.
4. Make a solution of one (1) ounce of sodium or potassium metabisulphite and half (1/2) ounce of citric acid to a gallon of water. One (1) gallon is enough for a five (5) gallon barrel. Make two (2) to three (3) gallons of solution for larger barrels. Pour in the solution, close the bung, and roll the barrel about so that all interior parts of the barrel are contacted by the solution.
5. Rinse the barrel once more with clean water, so as not to have too much sulphite or sulfur dioxide in the wine. The barrel is now ready to fill with your wine.
USED BARRELS
Even though a used barrel looks clean and smells sweet, it should be thoroughly cleaned before use. Flush with clean water and treat with the metabisulphite solution as described in steps #4 and #5.
There may be others methods of cleaning barrels - this is just one option. 
