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Personally, I have found that decals don't yellow unless they are in an environment where somebody smokes or have been in a bright light area. What I have found with my 30yo models is that the decals become brittle and disentegrate if left in the water for too long.
Trebor, you have some flat clear enamel or acrylic? Either use your brush or spray gun and put a thin coat of flat clear over your whole decal sheet. Cut your individual decals out. Soak them for 10-15sec max and put them on a paper towel/clean cloth for about a minute. Check it for ability to slide off the sheet. Don't manipulate the decal too much. Prep your model with a little decal solvent/set. Minimize your movement both on the sheet and on the model. Carefully blot with a clean soft cotton cloth.
Some long thin decals may break. Don't panic. You can easily piece them back together with only a little patience.
Here is one that I helped my son with. The decals completely disentegrated when we first started. I brushed some flat clear on them and had only a few faux pas.
Good luck!!
Hi trebor i think you can download instructions direct from revell