40% of panel receiving a metal coating (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

The first two letters of five (40%) in PA [nel], containing (receiving) A + TIN (metal). A surface layer that develops with age and handling, especially on metal or wood. If you like the way bronze darkens or copper goes blue-green, you call it patina; otherwise you probably call it tarnish.