As I had suggested in post #15, I finally decided that I should check my 24 year old spare to see if it even held air. I pulled it out the other night to inflate it. I did a complete system test by using the tool kit air compressor which had clearly been undisturbed in 24 years. The tire inflated without drama, although somewhat slowly. It took about 4-5 minutes for the tool kit air compressor to inflate the tire to around 35 psi. I let the tire sit for a little over 24 hours and checked to confirm that there was no pressure loss.
The tire was a little scummy. I have been wiping off the tread and spraying it with silicone every year; but, the sidewall never got wiped down, just spray in the sidewall fold and that has accumulated 13 years of dust and silicone. First inspection showed no ominous bulges after inflation
Inspection of the tread, tread shoulders and the sidewalls showed no cracks or surface checking of the rubber.
The tire has 5 very small ribs along each sidewall. Close inspection showed that while sitting in the uninflated condition it looks like in places, the side of some of the ribs bonded to their neighbouring rib and when the tire was inflated it pulled the rib apart.
It looks rather ugly, but, the depth of the crack in the separated area appears to be no deeper than the depth of the valleys between the ribs so this appears to be just superficial damage. Certainly there is no penetration down to the tire cords. I think the ribs might be part of the folding mechanism for the tire because they do appear to be located right at the side fold
when the tire is deflated.
The tire deflated and folded back up sort of successfully. I say sort of because initial deflation made the tire an exceedingly tight fit into the carrier in front. In order to completely deflate the tire you really need to remove or partially remove the tire valve to get the tire to compress completely.
So, I think my 2000 spare remains a viable solution. Given the 13 year accumulation of silicone spray, the tread remains rather slimy so I definitely would be driving very cautiously if I had to make use of it. That slime may scuff off after a few kilometers; but, caution is appropriate anyway given what it is not a 'real' tire.
If you are thinking that your folding spare might be your back up plan, it would probably be appropriate to inflate it and make sure it holds air. Fully inflated will allow you to inspect the carcass for signs of severe aging and then you can make a more informed decision about whether it is a viable spare.
I do carry one of those tire plugging tools as a back up to my spare. If a tire plugging tool has become your primary back up plan, you might want to test or rethink the use of the tool kit compressor. The tool kit compressor took about 4-5 minutes to inflate the rather small collapsible spare to 35 psi. The warning on the side of the compressor indicates a maximum run time of 30 minutes at which point you are to shut it off and let it cool down. If you need to inflate a rear tire and you are using the OEM 40 psi inflation pressure you could be in for a long wait to get it completely inflated with the tool kit compressor.
