Paint Depth Gauge
Although expensive, paint depth gauges can keep an eye on the thickness of paint. The gauges can guide you and display when the paint is too thin to continue wet sanding or even cutting with compound. Think about it: $400 for a depth gauge or the cost (not to mention labor) of repainting the panel that's too far gone to fix.

THE SHOPPING CART
ITEM PN PRICE
Cutting compound M10501 $30
Polishing compound M8232 $15
Wool pad W5000 $30
Foam pad W8000 $14
Detailing spray M3416 $7
TOTAL $96
SOURCE
Meguiar's Inc.
17991 Mitchell South
Irvine
CA  92614