How to tell if your blast profile meets the specification
Factors which have an effect on surface profile
The pitfalls of surface profile
Excess Profile – While an absence of profile can be detrimental to coating adhesion, it can be equally disastrous to have an excessive profile height causing premature rusting and coating failure. In addition more profile means using more paint to cover the surface! Consider these cases…
Rule of Thumb #1: Profile height should not exceed the primer coat DFT (Dry Film Thickness). Rule of Thumb #2: Profile height should not exceed the total coating system DFT. Embedment – Embedment of abrasive particles in the surface is a threat posed by friable, irregular shape abrasives. The embedded particle or fragment can stand out as a ‘rogue’ peak above the surrounding profile and may protrude through the applied coating.
Measuring surface profile
Using the Replica Tape Method to measure average maximum peak-to-valley profile height (Rz)
• Select a representative test site free of dust, dirt and pitting. • Choose the appropriate grade of Testex tape – refer to Inspection Instruments for details of the various scale measurement ranges. • Peel a test tape strip from the roll – a ‘bull’s-eye’ marker dot will remain on the slip paper. • Apply the tape to the test surface – rub over the tape with a finger to ensure it's firmly adhered.
Perform the Test
• Using moderate to firm pressure, rub the test window with the round-tip burnishing tool – refer page <?>. • Take care not to dislodge the test tape (caused by bumping the tool against the edge of the circular cutout window). • Burnish the test window until it has uniformly darkened – the color indicates the profile has been impressed into the test tape. • Peel the test tape strip from the surface.
Measuring the Test Result
• Use a dial thickness gage with the correct specifications (i.e. accuracy, anvil spring pressure and anvil size) for replica tape – refer to Inspection Instruments, page <?>. • Clean the anvils and check/adjust the zero point. • After cleaning and checking the gage zero point, adjust the dial to minus 2 mils (50 microns) (this compensates for the thickness of the tape carrier film and allows the profile measurement to be read directly from the gage). • Centre the test tape between the anvils, gently allow the anvils to close on the tape, and note the reading on the dial. • Take several readings to establish accuracy. (Reposition the tape in the anvils between each reading).