To mask aircraft canopies, I always use Cheap Chocolate Foil instead of expensive Bare Metal Foil. Apart from its low price, I think it has the perfect adhesive strength for this particular task, compared to the very strong adhesive of BMF. Since it is pretty difficult to describe CCF in words, I made the following photo report. Credits for the use of CCF go to my modeling friend Eric Verschuur!
It could be different in other parts of the world, but in Europe, chocolate bars are usually wrapped with two layers: aluminium foil and a printed paper wrapping. You can see the aluminium foil at both ends of the bar. Alternative sources are candy bars like Kit-Kat, candy rolls like Mentos, gum wrappers like Wrigley's and pepermint rolls.
You slide the printed paper wrapping off, and fold open the aluminium foil to reveal the contents.
This is what the foil should look like: matt whitish on the inside, bright and shiny on the outside.
The foil can be split in a thin aluminium layer and a paper layer. It's a fiddly job to make a start, and the splitting is relatively simple with some types of foil, and near impossible with others. You may have to shop around a little. Rich Cox found the perfect way to separate the foil from its backing: "Press an iron on the aluminum side for just a few moments. Find a temperature setting that doesn't make the layers split on their own. This warms up the waxy adhesive just enough that the two layers are very easy to separate, as one full sheet." I tried it, and Rich's technique works very well. If you do the ironing with a higher temperature setting, the layers will split on their own, and for some reason the tackiness is greatly reduced. Try it yourself and find what works best with your foil.
The split layers. The aluminium layer remains slightly tacky after the split. I think the adhesive is a wax of some sort.
CCF is perfect for masking canopies. I use a cotton swab to rub it on the canopy. I don't worry at all about folds. I neither worry about using more than one piece to cover a canopy. After the canopy is covered completely, I use my sharpest knife to make a very light cut along the frame lines. The foil sometimes lifts when you pull away the excess, but it is easily reattached with the cotton swab. I sometimes make small touch-ups with Maskol.
You can now paint the model. An Esci F-104G is shown below as an example. After painting, the foil is easily removed. It leaves a waxy residue, but this is cleaned off very easily with a tissue or a cotton swab. There is no sticky residue as with BMF.
And finally an example of the end result, a 1/72 Super Etendard.
CCF is less suited for permanent foiling. Because it was used to wrap chocolate, it usually has some folds that are nearly impossible to get rid off. It is also quite difficult to split a large piece of foil. I sometimes encounter spots where either there is no adhesive, or where the adhesive is so strong that some paper fibers remain attached to the foil. I guess BMF can't be beat for this application.