Actually, to be serious, (who? me? serious?) sometimes it's a matter of potluck as to what the decals in Tamiya kits will be like. I'm speaking mostly of their aircraft kits but I've noticed similar cases among the auto and armor kits which have passed through my hands.
As to what you can do to get Tamiya's decals to perform properly, I cannot tell you. I've never used any of 'em because I'm a firm believer that most kit decals, in general, are good for kindling for the campfire and nothing else. That's just me, though.
Those who have used kit decals, either out of necessity or preference, are the ones to provide the advice as to how they perform, and there's been a good bit of sound advice given in this thread already.
So even though I can't offer advice as to how to handle Tamiya's decals I can tell you that of all the Tamiya kits I've owned (speaking only of recent releases over the last ten to fifteeen years, that is) there have been three distinctly different 'types' of decals.
One type was what I refer to as the 'standard' type. Feli250, if the decals you were having problems with were the same ones as I used for my... uh... experiment (yeah, that's what it was...) then most likely they are the 'standard' type. They look godawful thick on the sheet and have a funky satin sheen to 'em but are printed rather nicely and look too nice to cut apart.
Another type I've noticed are what I call 'garage' decals. They also look kinda thick, but not as thick as the 'standards', are on a funky paper and, although you
can see the decal film, it looks like the decals and paper are as one. They're not printed quite as nicely as the 'standards', either.
The third type are printed by good 'ole Scale-Master. They're very thin, and are generally well printed, although they can be of wildly varying quality.
Anyhoo, my point is that Tamiya, like most any other model manufacturer, uses a number of different companies to provide decals for their kits. Because of this some techniques and decal setting solutions which work for one type of decal may not work the same way for other types.
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