Repair Apoxie clay on old resin

The age of the doll shouldn't matter much. Polyurethane doesn't change much over time apart from some superficial oxidization, as far as I know.
Just make sure the surfaces are clean and you should be fine.

 

I can't comment on how sturdy Aves' Apoxie Sculpt is under pressure, because I've ever only used it for facial mods. I have seen them (Aves) showing photos of customer's repairs on fountains, so I assume it is quite strong but I'm not personally experienced with it under constant pressure.

I can say that it adheres equally well on old resin as it does on new resin. I've recently modified a bunch of my older dolls and head sculpts, my oldest from 2007. As well as a newer 2022 doll's face in late 2024. So I can confirm Aves Apoxie Sculpt will work on old resin as well as the newer resins.

If you were wondering about other epoxy clays besides Aves, then I am afraid I'm only familiar with Aves' Apoxie, but I assume most will work on any aged resins just as well. I assumed you were wondering about Aves' specifically because theirs is the one named Apoxie, but some people believe it to be a missespelling of epoxy, so I was wondering if this was a typo, myself. Regardless, hope that helps and good luck! (:

 

I've never noticed much difference between brands when it comes to how well they adhere. Out of the ones mentioned I've used Aves Apoxy as well as Milliput and in addition to those I have experience with various brands of green stuff and epoxy filler in putty form. Over all, it's the same stuff and works about the same. Some have a finer texture, some are more gummy and springy to sculpt and some cure a little faster or slower, but over all they produce the same end result. It's mostly a matter of preference.
So I would assume that Magic Sculpt behaves more or less the same way too.

About restringing narrow channels, anything that can guide the elastic through the channel will work. Stiff ribbon, thin wire, fishing line. Whatever works.

 

I have done a similar repair and I am a big fan of using Apoxie Sculpt Super white to do this type of repair as well as fixing other chipped areas as it cures faster then the stone white and you can tint it to get it to a closer match to the resin color.