For hot dip galvanizing applications, the steel surface profile can be measured and verified in accordance with ASTM D4417, Standard Test Methods for Field Measurement of the Surface Profile of Sandblasted Steel, prior to application of repair materials when the surface has been prepared by power tool or abrasive blast cleaning methods. It is also possible to measure the surface profile of the hot dip galvanized coating prior to paint/powder coating application to verify surface preparation according to the product technical data sheets and paint manufacturer's instructions.
Although the standard for measuring surface profile is not directly referenced within the ASTM A780, D6386, or D7803 post-galvanizing specifications, they are referenced in the SSPC SP5/NACE No. 1 White Metal Blast Cleaning surface preparation standards. , SP10/NACE No. 2 Near-White Blast Cleaning, and SP11 Power Tool Cleaning.
Recent changes to ASTM D4417 provide updated test methods to reflect the latest advances in technology and new industry-accepted research. The three test methods described in the D4417 standard for measuring surface profile include visual comparators, surface profile depth micrometers, and replica tape. Only the use of surface profile depth micrometers and replica tape has been updated, and the specification changes are as follows:
Visual Comparator - This test method, as well as the comparator discs available on the market, have remained the same, and therefore there have been no updates to this test method.
Surface Profile Depth Micrometer – The maximum individual reading of 10 or more surface profile readings per area is now required to be reported, rather than taking the average of ten readings per area. Larger repair areas or hot-dip galvanized surfaces prepared for painting can be measured with 10 readings taken from a minimum of three 6x6 inch (15x15 cm) areas. The standard does not address discarding outliers, but an update to the standard addressing outlier readings is planned. These changes reflect industry research into current tooling, which has revealed that averaged measurements are consistently lower than values obtained with replica tape. Instead, recording the highest reading when using a depth micrometer is the one that most closely approximates the same measurements produced with the replica tape.
Replica Tape - Although the technology has remained virtually constant for replica tape since its introduction, industry research has addressed the sufficient number of readings necessary to achieve an accurate measurement. As a result, it is now only necessary to average two readings per area to obtain a measurement, instead of the three readings previously required. For measurements that fall between tape sizes, two readings are still required to be averaged with each tape to obtain one reading (i.e., 4 total readings are required for these cases). Larger repair areas or hot-dip galvanized surfaces prepared for painting can be measured with two readings taken from a minimum of three 6x6 inch (15x15 cm) areas.
