Very Old Decals

You can stabilize the decals if they are still on the sheet by overspraying them with glosscoat or dullcoat. This creates a new carrier film and prevents them from dissolving in the water. You do have to trim them more closely since you now have a continuos decal sheet and not individual decals.

microscale makes a decal film that'll help too. It can be brushed on. If the decal sheet has yellowed putting it in a sunny window for a while can bleach the yellow color out.

Thank you all

Old Soldier
Another problem you may run into with old Micro Scale decal is that they won't release from the backing, no matter what you try. I've had two different sheets do that to me (the P-39 and P-40 1/72 sheets). Prior to those two sheets, the only problems was with disintergrating decals which were cured by the decal coat. One note, you need to put at least 3 coats and still treat the decal very carefully when you apply it. As far as I know, there is nothing you can do about the decals sticking to the backing paper except throw them away.