Let me add to the chorus of praise for "girlwithatruck"! What a great idea! My dash was trashed, I was going to buy a dashmat or something to cover up the rot.
I hadn't considered repairing it, but this thread tempted me to try.
My dash was so rotten, I cut out all the perforated material, and was left with a big, 8-sided speaker hole. One place where I diverged slightly, was to trowel a layer of fiberglass window screen inside the expando-foam layer, hopefully to give more structural strength. I also used cheap Valspar "plastic spray paint", from Lowes. (Supposed to bond with plastic, no primer or prep needed?)
I'm repainting all the interior trim, door panels, etc.
Well, I started last night, and just finished buttoning up the dash. That includes painting about a dozen trim pieces, and replacing all the instrument panel lights (while I had it out). A warm sunny day, the spray paint dries in about 15 minutes. What an outstanding one-day project!
I took a few pictures, but can't seem to find my camera cable

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Thanks again for a great idea! This is an easy job that yields a big improvement in looks for your truck interior!