Decal Restoration.

Note that there is a decal forum just below the techniques one.  Lots of good ideas in the posts there.

Or weeks but it works well. Avoid condensation as well.

When applying a clear coat be aware that you then have to cut the decal out, which is generally pretty easy.

Hang from a window in direct sunlight for a week.

I've had good luck with Microscale superflim brush a thin coat give it an hour or so to dry then apply decals as normal, this works good for cracked decals. To get the yellow out lay the decals in sun light for a few days that will bleach the yellow out. I once did an old Stuka kit I used the yellowed decals after the decals had dried gave a coat of Testors dull coat the yellowing dissapeared.       ACESES5