I was recently asked by a fellow model builder if I had any advise on how to use a decal sheet that was from a kit from 1977. The kit plastic had survived 40+ years but the decal sheet had yellowed. Though the decals had not cracked, faded, or show other signs of deterioration, it was unlikely they would survive the application process.
Working with many vintage kits that have old decal sheets, I have had some success with restoring, recreating and using the old decals.
If the decals have yellowed, I will place the sheet in sunlight for a few hours a day. This helps get rid of the yellow many times. I don’t leave it all day as I do not want the decals to start to fade, just the yellow. Keep the sheet somewhere you can keep an eye on it. I have taken sheets to work with me in the past.
I will also scan the decal sheet into my computer so that I can try to print them on new decal paper or be able to cut my own masks to paint on the larger markings. Airbrushing a light coat of clear gloss lacquer or Testors decal spray to the whole sheet will help strengthen the decals before applying them into water.
When ready to apply, I will test first with a small decal. If there are marking for more then one version on the sheet I will take a decal from the markings I am not going to use. I will try it on a junk kit or an old water bottle or some other smooth plastic surface to see if they will survive handling and decal solvents.
When it looks like the decals will work, I will trim away as much of the clear carrier film away from the decal so that I do not have to deal with that part of it. With larger marking that I do not think will handle the application process as a one piece decal, I will find a place on the decal that will follow a panel line on the model and cut the decal to follow that panel line if possible. Then I only have to deal with two smaller decals instead of one. I can go back and with a little paint and fine brush, fill in the gap if any where I cut the decal.
If all else has failed then its usually time to check my spares, print or paint and last, look for aftermarket decals.