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I too am working on fixing cracks in one of the two spare dash shells I have.
(By the way if you want to spend big bucks there's an Aussie place that will totally redo your dash with new foam and new vacuum-formed black leather-grain vinyl shell for like $500, shipping included from a Washington state US affiliate. Do a search under dash repair on the Classic 900 bulletin board.)
As I get to it I am filling around a dozen cracks, starting with sticking a stiff blade down inside to crush back some of the foam to give a stronger wider deeper keying to my filler material. I'm using 3M DP-420 epoxy, mixed with a tiny dab of black oil artist's paint to come close to the color. Doesn't seem to affect the set or bonding strength of the epoxy.
I have despaired of getting any kind of graining in the top finished coat and instead intend to sand it carefully to a flat black and figure if I keep the epoxy off the nearby dash the cracks won't catch the eye too bad.
A problem I didn't think of and address at first is, what if the cracks want to continue? Doing it again I'd recommend first drilling a small round hole just at the point of each crack, to sort of dead-end the stresses, something advised also on stress-cracked steel panels etc.
Doing what I'm doing I have to fill the cracks in many steps as it's hard to get a lot of the epoxy in each time. Then I have to use a rag and paint thinner (mineral spirits seems to work fine) to clean excess epoxy off the top of dash.
As I neared the top I notice that the edges of almost all the cracks tend to want to curl up so there'll be a more noticeable ridge when I'm done. Before beginning you might want to experiment with a heat gun on softening the edges and trying to cancel this curl. (It's too late for me as epoxy already firmly adhered to it.)
Once crack is nearly filled I think I can pretty close to control spreading a final layer of epoxy up to flush with crack edges, using a flat toothpick as spreader. As it's nearly set, wiping with mineral spirits seems to flatten the gloss of the finish so that's my plan, then sand any problem areas.
It might also be possible to fill cracks to near top with epoxy and then use one of those vinyl repair kits and graining paper to make the top layer come out better.
A guy once posted here wondering about treatments for the dash to forestall cracks, and said he'd heard of using Vaseline. He was hooted off but I think it might have some value, as re-imparting to the black vinyl a "plasticizer" to give it back flexibility and stretch.
The idea being, apply it like a mud-mask facial and let it good and soak in say for a few days in the hot sun. Then wiping it clean and even washing with soap and water or mild detergent would probably take away the objectionable oily gleam but leave a fair amount in the plastic.
I don't know chemistry but I think it's fair to think of most plastics as like a sort of sponge with an oily plasticizer component included that lends flex, kind of like a little water dampening a cellulose kitchen sponge. As that plasticizer evaporates, oxidizes and is washed out by coffee spills, soda spills and repeated cleanings, the plastic gets more brittle and less able to stretch under the sun's heat stress, which is why the cracks.
In fact I used to know the name of one of those plasticizers that they warned about being in children's pacifiers and coming out as they chewed, with potential carcinogenic properties. It would probably be dangerous to handle and apply if you could get it so scratch that.
posted by 64.223.210...
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