Important Information from ASTM A 123/A 123M
Extracted, with permission, from A 123/A 123M-02 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron

(3.2.8) Specimen: The surface of an individual test article or a portion of a test article, upon which thickness measurements are to be performed, which is a member of a lot, or a member of a sample representing that lot. For magnetic thickness measurements, specimen excludes any area of the surface which is subject to processes (such as flame cutting, machining, threading, etc.) that can be expected to result in surface conditions not representative of the general surface condition of the test article, or is disqualified by the measurement method.

6. Coating Properties
(6.1) Coating Thickness: The average thickness of coating for all specimens tested shall conform to the requirements of Table 1 for the categories and thicknesses of the material being galvanized. Minimum average thickness of coating for any individual specimen is one coating grade less than that required in Table 1. Where products consisting of various material thicknesses or categories are galvanized, the coating thickness grades for each thickness range and material category of material shall be as shown in Table 1.
(6.1.1) For articles whose surface area is greater than 160 in.2 (100,000 mm2) (multi-specimen articles), each test article in the sample must meet the appropriate minimum average coating thickness grade requirements of Table 1, and each specimen coating thickness grade comprising that overall average for each test article shall average not less than one coating grade below that required in Table 1.
(6.1.2) For articles whose surface area is equal to or less than 160 in.2 (100,000 mm2) (single-specimen articles), the average of all test articles in the sample must meet the appropriate minimum average coating thickness grade requirements of Table 1, and for each test article, its specimen coating thickness shall be not less than one coating grade below that required in Table 1.
(6.1.3) No individual measurement, or cluster of measurements at the same general location, on a test specimen shall be cause for rejection under the coating thickness requirements of this specification provided that when those measurements are averaged with the other dispersed measurements to determine the specimen coating thickness grade for that specimen, the requirements of 6.1.1 or 6.1.2, as appropriate, are met.:

TABLE 2: Coating Thickness Grade

Multi-specimen articles
Articles whose surface area is greater than 160 sq. in. (100,000 sq. mm)

Each test article in the sample (3 specimens each) must have Minimum Average Coating Thickness of Table 1.

Each specimen (5 or more measurements widely dispersed) must have Minimum Average Coating Thickness of Table 1 minus one grade.

Single-specimen articles
Articles whose surface area is equal to or less than 160 sq. in. (100,000 sq. mm).
Each specimen (5 or more measurements widely dispersed) must have Minimum Average Coating Thickness of Table 1 minus one grade.
All test articles (specimens) together must have Minimum Average Coating Thickness of Table 1.

Selection and number of test specimens
(7.3) The method of selection and number of test specimens shall be agreed upon between the galvanizer and the purchaser. Otherwise, the test specimens shall be selected at random from each lot. In this case, the minimum number of specimens from each lot shall be as specified in table below.