If you have cracks or gouging then just add liquid super glue, not gel to the area then immediately cover it with baking soda. It hardens like a rock that can be sanded and painted.
I have an original pair that need some restoration. I'll try the super glue and baking soda idea. Seems like I might have to do it a couple of times to build up thickness. What material do you suggest I work on the will not stick to the super glue or should I just figure on sanding off the paper or whatever that I work on. Thanks for the tip. I've been meaning to ask for ideas on making these repairs.