How can I soften and remove marine spar varnish from metal without removiing the paint underneath?

Question

There are 2 coats of marine spar varnish on the black metal door threshold at my front door that was applied by renters 2 years ago. The varnish is starting to blister but it is difficult to peel off. I tried Goop Off and paint thinner and neither did anything.

Answer 1

I would try Mineral Spirits. Goo Gone or Goof Off will also remove the paint.

Answer 2

I would try using acetone first, and if that didn't work I would try muratic acid (be careful using that stuff). If it were my place, I would simply apply a new coat or two of fresh varnish over it.