I stumbled across this fix and thought I'd share.
I recently installed a sunbird turbo tach that I purchased on ebay from a gentleman who was parting out his old 87 junker. The pictures on the listing weren't the best, but it wasn't a bad deal considering that I'd be getting the entire gauge cluster, so I went for it. Unfortunately, once I got the cluster I saw that the faces of the gauges were pretty faded from sitting out in the southern sun. The speedometer got it the worst, but the tach was faded to the point where it had begun to turn light grey/white toward the center.
I initially toyed with the idea of printing a gauge overlay to hide the fading, but my results just weren't quite what I was looking for. It was at this point that that the lightbulb came on. What about putting a coat of clear satin/semigloss paint on the gauge to revitalize the faded colors? After testing my idea on the speedometer with some krylon gloss clear that I had lying around, I was happy enough with the results to clearcoat the tachometer.
'87 GT 3.4 pushrod daily driver
'88 GT 3.4 DOHC swap in progress