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Wojtek, the Liberator looks great! Can you get these card kits in the UK? I quite fancy trying one...
As I mentioned in your other thread Rob, the decal papers from Testors are more or less the same as from Bare Metal and other suppliers (probably the same stuff anyway!). As long as you use the correct paper for the printer, that is, either ink jet or laser, then they should work.
Make sure you don't touch the sheet once it comes out of the printer until it has had time to dry, as it takes a bit longer than 'normal'printer paper, and can easily smudge. Spray the sheet with a clear, acrylic varnish before use, making sure it's a light coating, which can be repeated if required once the first coat has fully dried.
Leave this for at least two hours, and preferably overnight, for example, to let the varnish harden and seal the image, and then use as normal decals, Don't forget, you can not print white - any areas with white in them will come out clear on the decal, unless white paper is used, and the colour density will be nowhere near as dense as on commercial decals.