well you have to ask them if they think they can do it. if it's brushed alu, then you run the possibility of sanding through the decal and "brushing" the aluminum the wrong way. and at that point in becomes hard to fix.
a good painter will have a portfolio to look through so you can judge for yourself if you think they should do it or not. it's actually kind of a tough call.
ask if they can do it and what happens if they mar the metal on accident.
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