Just ran across this here. I've printed on metal for several years - aluminum, stainless steel, and copper - using InkAid for the coating. Metal purchased from a supply house has been a lot easier for me than working with flashing. I found a local place that supplies metal to roofers and light manufacturers. They'll cut to specification, and that means you can select a big sheet of exactly the surface type and thickness you want, specify the sizes you want cut out of the sheet, and go home happy with metal plates that cost a fraction of buying pre-coated metal. The one problem with this approach is that the metal distributors are not used to people wanting perfect, unscratched, unmarred surfaces - they assume if that matters, you'll be buffing and surfacing yourself, and that's a total pain in the posterior, especially with stainless steel. Metal polishing is an acquired skill. By doing a lot of begging and whining
I've been able to get the shop to be more careful, and now I get mostly perfect surfaces with a few pieces marred, and I use those pieces for experiments in texturing the metal before coating - swirls or texturing that follows the shapes of the image I'll be printing later.
There are bonuses for finding a local metal distributor to buy from. They have a lot of scrap pieces from other orders that are pretty good sized, although not to a specific perfect size, and if you're fine with that, they'll usually GIVE those pieces to you, or sell them for what a metal recycler would pay. About half of what I print on now is scrap - I put a standard image size on the nonstandard size pieces, even in some cases on geometrically odd pieces which can be really lovely. The other bonus is, odds are they've never had an artist customer. The local shop was fascinated at what I told them I was doing, and when I brought in some examples of printed images, they bought two to hang in their offices. I've since given them a couple more to display, and the result has been some sales of my images on metal to their customers.
I've also worked on aluminum flashing purchased at the home improvement stores, initially because I had some large panoramas I wanted to try on metal. (14x144 inches.) While both purchased metal sheets and flashing require pretty finicky cleaning - I use TSP if the surface is really slick, metal cleaners and solid sponges at minimum - the flashing seems to be the trickiest to prep. I wish I knew why, but it just is.
When I coat with InkAid, I do two or three coats usually, with 24 hours of drying time. I use brushes since I don't have an industrial sprayer. My biggest issue is getting that many coats on huge surfaces over several days without accidentally getting a stray piece of dog hair or dust in the surface. I have a room in the house now that I have sealed, that I use for coating, which helps a lot. After printing, I wait at least 24 hours, then coat with a couple layers of one of the spray coatings, the nasty smelling varnish types. I tried NOT doing that, and the surfaces were incredibly touchy. If I spray coat, they're not much more touchy than some of the Hahnemuhle papers.
Some day I am going to try the pre-coated metal options, because for work I sell, the extra cost is well covered by the price of the prints I sell, and I'm a photographer, not a print surface maker. I'd still use my own coated surfaces for proofing and tweaking prints, but once I had what I wanted, I can wash off the metal, re-coat, and re-use for proofing again.