Rejuvenator is a dope with strong solvents added.
It will almost work like any glue to seal the ringworm for about 6 months, then will crack again because cracks are from thick dope finish.
Insstead, I would glue nice round fabric patches over the ringworm spots, if minor. Or recover, if major.
Used it a lot in ag world. Used to joke at the term "fabric rejuvenator" because the only thing being rejuvenated is the finish, as that's is what has failed. What has been said - solvent softens the finish so the imperfections can flow out while new finish coats are applied. How well it works is mostly technique. Have to sand most of the failed finish off or the failure will repeat in short order. Also requires lot of sanding with 600 and rub out when cured.
Poly tone rejuvenates fairly well, should not be able to tell a repair was done once complete.
Rejuvenator is a slow-drying blend of solvent and plasticizers that will make the old coatings supple and flexible again - but it won't change the appearance much.
To fix existing ringworm, get a brush and a quart of Poly Spray, and get some of the silver mud off the bottom of the can. Brush it into the ringworm and build up some silver. Now wet sand the excess silver until it is smooth.
Then spray with rejuvenator - mask off anything metal or plastic, and don't do it in really cool weather because the slow drying solvent can load the surface with vapors and risk popping glue joints.
After the rejuvenator dries, you'll have a nice, flexible coating again. If you want the coating to look new, you'll have to spray another cross coat of Poly-Tone on top. It's not a small job, but it's way less work and cost than recovering.
The drawback to rejuvenating is you don't get a chance to inspect the airframe, and it doesn't add any longevity to the fabric itself. If the coating have deteriorated enough to let UV damage happen to the fabric underneath, then the rejuvenation process will do nothing except put lipstick on a pig.