The 'E' Type Forum

When repainting a car how important is it to go down to bare metal? Does the existing paint have a life span beyond which it should not be overpainted?

It's more about removing all the paint and any filler or other material prior to refinishing the panels to get the best possible surface and profile prior to painting.

Existing paint or filler may have corrosion hidden under it that you simply can't see which may bubble after you've painted the car. It can react with anything you spray over it, and may not provide as good a surface for your paint to key to.

Going "back to bare metal" takes time and costs money but it's the best way to get a long lasting quality finish.

Also, depending on the paint used it may be required.
For example you can't paint lacquer or straight enamel with urethanes-they lift.