10/31/2015 Edited to add: Since I wrote this I found Rainbow Dust paints and glitters, which are totally edible and give you a really good shine. This post contains affiliate links.
There are a lot of photos of metallic cakes floating around pinterest etc these days. The metallic finishes are done with gold or silver luster dusts that are applied to fondant, but if you don't put them on right they won't look as metallic as you want them to.
I've done a lot of cakes with metallic details, but I don't think that I'd even done one that was as intentionally shiny as some of the ones that I've seen recently. I asked one designer how she had done the finish on one of her cakes, and she said that she used a big brush and diluted gold luster dust.
I wasn't sure that would work, since I was thinking about the brush marks, so I tried it, plus a few other methods, to see which would be more metallic. Keep in mind the non-toxic vs. edible issues, too...Some gold dusts aren't even non-toxic, so tell people not to eat things that aren't labeled as edible.
I used three pieces of gumpaste, vodka and gold luster dust. The brush was a soft one that's similar to a puffy makeup brush, so it wouldn't leave edge marks. I also used one piece of candy clay because the oils from that usually give a better shine than fondant or gumpaste does.
Now...After these trials, a thick coat of the luster dust mixed with vodka did give a good shine and deep color. But there's one missing element that hadn't come up in any of the discussions about the technique that I'd had, but I noticed when I started looking at all those photos. The lighting.....
Notice the difference in the strips that I was holding up in the photos? The light coming from the nearby window totally transformed the gold from flat to metallic and shiny. You can paint a cake gold, but if the lighting isn't right it isn't going to look as metallic as it will if there's a light shining on it.
This is actually something that I warn brides about if they want silver on their cakes. Without the right lighting, silver metallic details can look grey, not silver. With a camera flash the details on metallic cakes might make the photos look a lot more metallic than the actual cake did. If the bride is okay with pewter, not bright silver, this might be okay, but I always warn people anyway.
Using this gold cake as an example, look at how the camera flash and the lighting is reflecting off of the metallic and makes some parts of the gold look brighter. This makes the entire cake look metallic because your brain fills in the blanks and tells you to see the entire thing as metallic if part of it is reflecting that way.
So I think that the key to getting a very metallic look to a cake would be to use a thick coat of the luster dust in alcohol, and then make sure that the lighting in the venue is conducive to enhancing the shine. Suggest that the bride have a lighting person put a pin spot shining on the cake table if she wants a super reflective, metallic surface. If the room is too dark it will look gold, but it won't shine.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA, and cake supplies online at www.acaketoremember.biz and www.acaketoremember.etsy.com