imparts more tension to the fabric and thus requires a lesser number of coats. Cover with a semipigmented dope as soon as practicable to match surrounding finish. However, patches applied with this material are to be used in emergency only. Semipigmented dope should be used when initial time saving is not essential. 23. Treatment of Parts.      a. Re-used Steel Parts. - Steel parts which are removed and are to be re-used should be treated as follows:      (1) Remove all paint, grease, and oil by immersing in 20 percent sodium hydroxide (NaOH) 80 percent water solution. This may be performed at room temperature, or in a warm solution (faster), and in general should not require much more than 10 to 15 minutes.      (2) Remove cadmium, rust and corrosion with a 60 percent hydrochloric acid (muriatic) 40 percent water solution. This must be watched to prevent overpickling. In many cases a single momentary dip will suffice.      (3) Replace with .0005-inch minimum thickness, using standard cadmium plating practice.      (4) Cadmium plated parts (and also unplated parts), which require painting shall be chromic acid dipped for 2 to 5 minutes in a water solution at room temperature, containing 3 to 5 percent chromic acid, rinsed and dried.      (5) Paint.      b. New Steel Parts. ~ New steel parts should be treated as follows:      (1) Clean with any approved alkali cleaner either in hot or cold solution and rinse in water.      (2) Pickling to prepare for plating may proceed in a 25 percent hydrochloric (muriatic) acid bath for 5 to 10 minutes. Steel springs shall never receive more than a momentary dip.      (3) Plate with .005-inch minimum thickness,using standard cadmium plating practices.

(4) Cadmium plated parts (and also unplated parts) which require painting, shall be chromic acid dipped for 2 to 5 minutes in a water solution, at room temperature, containing 3 to 5 percent chromic acid, rinsed and dried.      (5) Paint.      c. Aluminum Alloy Parts. - Aluminum-alloy parts should be treated as follows:      (1) Remove paint, grease, and oil by dipping in a suitable approved inhibited mild alkali cleaner, and rinse in fresh water. Cleaning may be aided by hand srubbing. Parts excessively oily or greasy may be given a standard degrease before this alkali cleaning.      (2) Dip for 2 to 5 minutes in a water solution, at room temperature, containing 3 to 5 percent chromic acid, rinse and dry. After this treatment, and before painting, parts should be handled as little as possible.      (3) Paint.      d. Magnesium-Alloy Parts. - Magnesium-alloy parts should be treated as follows:      (1) Rough sand castings or other parts which are excessively dirty or which contain imbedded bold sand etc., may be cleaned by dipping for 5 minutes in a 15 to 20 percent hydrofluoric acid (HF) 80 percent water solution. A satisfactory bath can be prepared by diluting one volume of technical grade 48 to 52 percent (HF) with two volumes of water. Wash thoroughly in cold water.      (2) Boil casting for 45 minutes in a water solution containing 10 percent by weight of sodium dichromate (Na2Cr2O7). This solution can be readily prepared by dissolving technical sodium dichromate in water in the ratio of 3/4 pound per gallon.      (3) Depletion of the 10 percent sodium dichromate solution will be indicated by non uniformity of coating. It can be crudely revivified by adding 1.3 percent or less of chromic acid. More positive control can be accomplished by adding sufficient chromic acid to bring the pH value of the bath to 4.2.      (4) Dry in an oven and paint immediately.

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