Decals, Shelf Life, and Other Issues.

[SfanGoch]

But that water-soluble glue you mentioned is evenly applied to the entire decal paper sheet (not just under the film of each decal image).  IMO, it is the clear film itself that yellows (not the adhesive).  Or maybe the adhesive reacts with the clear decal film causing yellowing?

Like you, I had success bleaching the unused yellowed decals using sunlight. I have not tried UV-bleaching the ones applied to models.

 

UV radiation is a component of sunlight; but, I don't think it'd be a good idea to leave a built model sitting in direct sunlight for any extended period of time. I dunno.....maybe if you got a No! No! Pro laying around, that might work.  

I originally was going to bring up the quality of the items used to manufacture decals when I commented that all decals aren't created equally. Art took care of that. 

[2002p51]

I think smoking would have yellowed the entire model, not just the decals.

I agree and, just for the record, my wife and I are both non-smokers. 

What really surprised me about this particular model was the speed with which those decals turned yellow. I've got models on the shelf built in the late '60s and early '70s that have exhibited some degree of yellowing. But this model was built and these decals used LESS than two years ago!

[peteski]

I agree and, just for the record, my wife and I are both non-smokers. 

What really surprised me about this particular model was the speed with which those decals turned yellow. I've got models on the shelf built in the late '60s and early '70s that have exhibited some degree of yellowing. But this model was built and these decals used LESS than two years ago!

It could just be due to whatever substance they used for the clear decal film. Maybe it is simply prone to yellowing when exposed to air or it slowly reacted with  the  clear-coat on your model.

[SfanGoch]

But this model was built and these decals used LESS than two years ago!

You did mention in the original post that those particular decals were 20 years old. Ageing affects everything in one way or another. Have you taken a look at Tina "Ginger Grant" Louise lately?

[Daddyfink]

Most decals yellow due to the type of varnish that they are made with. It will yellow with time. 

[2002p51]

It could just be due to whatever substance they used for the clear decal film. Maybe it is simply prone to yellowing when exposed to air or it slowly reacted with  the  clear-coat on your model.

I never use clear coats over decals on race car models.