Here is a photo of an 84 year old 100k "body tip spot" resistor showing its still very accurate value today. You can just make out the spiral of what I assume is a carbon compound where the paint has chipped away.
The heater ballast resistor on this radio is mounted vertically near the back of the radio, I suppose near the back it can get maximum ventilation around it. But the heat has made the card back very brittle and hence some of the card has broken away leaving the back flopping around on one side.
This is a bit of an experiment but I have tried to repair this.
First I cut away the broken edges to give a clean right angle. The cut edges were still crumbling to dust so I used superglue (it needs to be the very thin runny type) to seal all the cut edges and the crack which ran down the back so that a strong bond can later be made with PVA glue. The super glue is absorbed into the dry card and it sets rock hard giving a firm base for the PVA.
I could not find any suitable thickness hardboard so I used a piece of thin ply to make the piece to fit in the corner. It was glued overnight and I also made a thin batten to help straighten a curve which had set in the back and strengthen the new ply I had glued in.
Next I have attempted to simulate the dimpled effect of the original surface of the back by covering it in PVA and then using a fine paint brush to stipple it to make it onto more like blobs.
I wonder what it will look like when dry and painted, hopefully a little better than the flat smooth plywood.
Mike