Fabric is flexible. Most auto finishes are not. If you must use auto paint, add a flex agent like they use for rubber bumper covers.
If the plane is finished with dope, you can rejuvinate the fabric (if required due to cracking of the dope) and refinish again with dope. You can use butyrate dope over nitrate, but not the other way around.
I think taking this to an automotive shop is a bad idea. Do't let them convince you otherwise, I don't care how good they think they are! Unless they are familiar with fabric aircraft, you better not let them do ANY sanding. They will sand right through the fabric over any stiffeners or ribs and ruin the fabric. All sanding has to be done by hand!
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99c827db9baac81b86256b4500596c4e/$FILE/Chapter%2002.pdf2-35. REJUVENATION OF THE DOPE
FILM. If fabric loses its strength, there is
nothing to do but remove it and recover the
aircraft. But if the fabric is good and the dope
is cracked, it may be treated with rejuvenator, a
mixture of very potent solvents and plasticizers,
to restore its resilience. The surface of the
fabric is cleaned and the rejuvenator sprayed
on in a wet coat, and the solvents soften the
old finish so the plasticizers can become part
of the film. When the rejuvenator dries, the
surface should be sprayed with two coats of
aluminum-pigmented dope, then sanded and a
third coat of aluminum-pigmented dope applied,
followed with the colored-dope finish.
When repairing, rejuvenating, and refinishing
covering materials approved under an STC, instructions
in the manual furnished by the material
supplier should be followed.