This is a cross post - What technique can you use to remove the yellow tint to old decals. I’m working on the old Aurora Wanderer Whaler and while the decals look solid, the white lettering is yellowed from age. My alternative is to use dry transfer decals from Woodland Scenes. The whale boats have black lettering, which I’m trying for verify they even put names on the whaleboats or was that something Aurora came up with.
Thanks.
Nothing, once they yellow they are trash.
Steve
Sometimes, this isn’t always a fix, you can put them in a window 
that gets a good amount of sunlight. Leave it in a spot in the window where it won’t get damaged for a few days to a week or so. I have had some that it worked on and I have had some that it didn’t. If it works, I would also spray it with a clear coat or a clear varnish. Many decal sets get brittle when they get older. The clear should keep the decals from splintering on you when you try to apply them. I hope this helps.