Possible Clues:

• Having a metal coating
• Covered, as with gold
• Coated, as with gold
• Gilt, say
• Metal-coated
• It's all on the outside, but dead on the inside
• It's only on the surface that it's dead inside
• Like chrome hubcaps
• Not in time inside, just on the surface
• On the outside it's valuable, but inside it's 30 across (6)