Fingernail test
Fingernail test turns up in 2 fields: powder coating, gears. Here is what it means to the people in each one.
Fingernail test in powder coating
The rough floor check of cure: dig a thumbnail into a cooled part and see if it marks.
Before reaching for the MEK, many sprayers press a fingernail into an inconspicuous spot on a cooled part. A soft, marking film suggests undercure; a hard one that resists suggests cure is close. It is fast and catches gross problems, but it cannot confirm full cure, and a warm part always feels softer than a cold one.
"Did the fingernail test on the bracket edge and it dug right in, these never got to temperature."
Fingernail test in gears
Shop check of dragging a fingernail across a tooth flank to judge whether surface damage has real depth.
A quick field assessment during inspections of open gearboxes: a fingernail drawn across a mark on the tooth flank either glides over it, suggesting polishing or staining, or catches, indicating a scratch, scuff, pit edge or step with measurable depth. It is not a measurement but helps sort cosmetic marks from damage that needs a photo, a replica or a closer look with a borescope.
"Fingernail test catches on the scuff marks at the tips, so it's not just polishing, log it and pull an oil sample."