Ok. I might see if my buddy Rex will let me have an old dash from the junk yard and I can test the Great Stuff, the Epoxy, and the other suggestions and see what fares the best after leaving it out in the weather under the car port. Couldn't hurt. And if none of it works, I'll buy a dash cap.

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1982 300SD Light Blue
2002 Honda Accord SE
1974 Toro Wheel Horse Tractor
2000 Toyota Tundra Pickup