I haven't actually tried it yet since the weather here has been too bad for this kind of project, but I have a filler made by SEM called "Problem Plastic Repair Material" that's supposed to work for filling dash cracks. I'm going to try cleaning up the cracks with a Dremel tool, filling them, going over the repairs with spray texture coating and then spraying the entire dash with SEM Color Coat.
I've tried three other types of plastic filler already and all of them cracked again as soon as the weather turned cold. I guess if the SEM filler doesn't work, I'll give up and get a dash cap...

-Hank

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1984 MB 300TD
2003 Subaru WRX