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This collier is much more than 10 years old. My wife wears it very often. It is made from Thompson copper balls. The inside is NOT enamelled and till now the enamel is not demaged.
If I make my own balls, I use only 0,5 mm thick copper. This, mostly irregular formed balls, I enameling also in the inside.
Edmund
Dear Trish,
I made a few days holidays at the beautiful "Middle-Rhine", a area full of culture starting about 130 000 years before Christus (found of one of the oldest human articulated skeleton in Germany found in the Neantertal near Düsseldorf) over all cultures with all the ruins, castles, Cathedrales etc. until to the modern architecture and art museums and the beautiful landscape with delicious wines and meals.
During this travel I visited the Cologne Cathedral again with the beautiful "Dreikönigsschrein" (Epiphany Shrine, in 1200 century builded) which enclose a lot of Champlevé-enamel plates. 
 And in the trasure room of the Cathedrale of Limburg a very modern "Evangeliar" builded about 1990 by the nun Lioba Munz. 
That the reason that I answer only now.
To your question. I think nothing about the use of resin to protect enamelled surfaces. The surface of enamel is much more immune about any environmental stress than the best resin.
And I think, the resin can not protect the enamel for chipping. That the enamel chip off is a mechanical problem.
Bangle Bracelets succumb a high mechanical stress if they are in use. Already minor bending stress let the enamel jump off.
With friendly regards
Edmund
Yes Trish, I know too, that very old enamels becomes demaged. In the "Grünes Gewölbe" in Dresden they .have a lot of problems with the more than 400-500 years old enamels. They try to restaurate it and they wrote an extensive report about all the problems. 
But in the architecture enamelled facades of houses apply as the most stable paneling.
I myself have made several garden-signs. Some of this standing since more than 40 years sommer and winter outside without any damaging.
To the Resin: I remeber the development of the resins after world war II. The first resins were very instable. Already after a few years outside, it got gray, crumbly and fall to pieces. Certainly the modern resins are much more stable, but I think in 400-500 years it gets much more collapsed than enamels.
I myself don't see any reason, for protecting enamelled surfaces with resin. Resin is so much softer than enamel, so that the resin-surface becomes much more quickly scratched and shabby, especially if one jewelery with resin clogging.
Edmund