MIL-P-38336A(USAF)
4.4. Test panels
4.4.1 Materials. Test panels shall be prepared from sheet steel conforming to QQ-S-698, cold-rolled, and may be of any convenient size and thickness subject to the following limitations:
a. Unless otherwise specified, panels shall be at least 4 inches wide by 6 inches long. b. Panels for the flexibility tests shall be 22 +2 mils thick.
c. Other test panels shall be not less than 30 nor more than 60 mils thick.
4.4.2 Surface preparation
4.4.2.1 Solvent cleaned panels. Test panels shall be selected from smooth steel free from rust. The panels shall be scoured with steel wool, washed in an environmentally acceptable cleaner. After the panels have air dried, they shall be wiped clean with a clean lint-free cloth.
4.4.2.2 Phosphoric acid cleaned panels. The test panels selected shall be free from other than light rusting. They shall be scoured with steel wool to remove rust or other adhering contamination, washed as in 4.4.2.1, rinsed and air dried. The panels shall then be treated as follows:
a. Immerse for two hours in an acid cleaner of the following composition maintained at a temperature of 75°F +2°F:
3150 ml of water
650 ml of technical grade, 85 percent phosphoric acid
3.8 grams of diethyl 1,3, thiourea
1.4 grams of alkylarylsurfonate wetting agent
b. Rinse in water and immerse for 15 minutes in the following dichromate solution maintained at a temperature of 75°F +2°F:
3785 ml of water
114 grams of potassium dichromate
c. Force dry the panels at a temperature of 190°F to 212°F
4.4.2.3 Sand blasted panels. The panels shall be washed as specified in 4.4.2.1, rinsed and dried. The entire panel shall be blasted using any suitable equipment and blasting material. The blasting material shall be free from oil, grease, dirt, water, or other contaminates that would impair the coatability of the panel surface. The panels shall be blast cleaned to a white metal. After blasting, the panels shall be cleaned by using clean, dry compressed air or a vacuum.
4.4.3 Application methods
4.4.3.1 Dry surfaces. After preparing the panels as specified in 4.4.2, the panels shall be kept clean and free from fingerprints, rust, or other surface contamination. Application of the primer shall be accomplished within a maximum of one hour after cleaning, or the cleaned panels may be stored in a desiccator for not more than 72
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