I used the Permatex adhesive from Advance Auto on mine. I ended up using a thin vinyl material to replace the original cloth. Wear a glove and gently brush all the old foam off with your hand prior to installing new material, you will do less damage to the backer board than using something like a wire brush. Mine was extremely fragile, but I managed to reuse it and it stayed in one piece. The instructions on the adhesive I used required a minimum of two coats in one direction, than two more in the opposite direction. You need to wait at least 5 minutes between each coat and do both surfaces. The stuff I used grabbed the material and would not let go once it was on, so get it straight the first time. It has been up for a month now with no signs of sagging. I completely removed the backer board and did it with the material facing up on my garage floor. I also allowed it to dry overnight prior to reinstalling. Hope this helps.

 

I've never had to deal with a foam headliner, though I did a 69 Chevelle with the fabric and ribs... challenging, to say the least.

I have wondered about possibly doing the cleaning, maybe finishing it up with light strokes of 80 grit sandpaper, then prime and paint. Even if it didn't look good, it'd seal the backing and might be a po' folks fix. The paint wouldn't prevent glue and fabric on top if it, too.

Just thinkin' (yea, dangerous,I know)...