30 year old water-slide decals. Are they still usable?

Question

Not strictly a drone or model aircraft question, but I have seen multiple questions here about painting models, and this is also about the aesthetics.

When clearing my deceased cousins house, I found several plane and car models he was in the process of building 30 years ago, but he never finished them.
In 1992 he got in a motorcycle accident, which shattered his right hand leaving him with severely restricted movement of that hand. He never got back into building models, because of the loss of dexterity.

As a tribute to him I would like to finish those models.

What has me worried are the water-slide decals.
There are a bunch of them to be placed over painted areas (Humbrol enamel paint).
They have been stored at room temperature in a closed (dark) cupboard all this time.
I have used some 5 year old decals in the past without any issues, but these are 30 years old.

Would these still be usable after 30 years or is it better not to try to put them on at all?

Answer 1

It's probably safe to assume that they will no longer work. This source source suggests that water-slide decals have shelf life of about 3 years. "After a few years the glue holding the decal onto the paper breaks down, fusing decal and paper together."

Because there is some variation between different decals from different manufacturers, they may last slightly shorter or longer than that in storage (like 3-5 years), but 30 years is probably too long. I think it's worth trying out one of the smaller decals just to be sure of whether it will work or not.

Answer 2

Posting a self-answer to tell you how I fared.
(Yes, it was a year ago... Took me a while to get around to this. Life got in the way.)

I had 8 old model-kits to put together. 4 of those were still unopened with the contents, including the water-slide decal sheet(s), sealed in plastic bags.
2 of these models were identical WW 1 era bi-planes. I think Mike originally intended to use these as part of a diorama showing a dogfight between these 2 British planes and a German plane (the Red Baron) that he already had completed.

Attempt 1
So knowing I had a spare for use in attempt 2 in case it would go horribly wrong, I used the decals of one of these planes as a test-case.
No luck: They failed to separate from the backing even after a very long soak in warm water.

So the sealed bags hadn't been enough to prevent degradation. Since all model-kits were roughly the same age and by the same manufacturer it didn't make any sense to try the others. Trying the decals that hadn't been bagged made even less sense.

Attempt 2
So I bought a pack of blank decal-paper for use in a laserjet printer. Put each original decal sheet on a flat-bed scanner and made a 600 DPI scan. Then printed each scan on the blank decal paper using the color laser-printer at the office.
Did a test-run on plain paper first to make sure the resulting print was exactly the right size and to verify how the colors came out.
I found I had to saturate the colors a little extra to compensate for the laser-printers slightly washed out colors.
After printing the first decal on the transparent decal paper I checked colors again and I found I needed to add even more saturation to get the colors just right.
Then I printed all the decals and I had a fresh set.

Conclusion
Copying the decals onto fresh paper turned out to be a very easy and cheap (€3,- for 10 sheets of A4) solution and I will certainly be doing this again when dealing with old decals.
If you are not certain about any decals that have been lying around for a long time it is certainly worth the effort to make a scan of them "just in case" you need replacements. Trying to scan them AFTER trying to use them won't go too well.
I had a flatbed scanner at my disposal, but you can probably get away with a smartphone photo of the decal. You just need to use some image-editing software to stretch the photo into a rectangle if you took the photo at an angle.

PS.
Blank decal paper is available for inktjet printers too. You will have to bear in mind that most inktjet inks are to a certain extend water soluble when they are not 100% dry. And it takes a very, very long time to really dry.
(I had inkjet prints blot when they got a few drops of water 6 months after being printed.)
Unless you are absolutely certain your inkjet ink will not smear when come in contact with water, using an inkjet to print your own water-slide decals doesn't seem a very good idea.

Answer 3

This is a big topic and a common problem, and you might have better luck in the dedicated scale modellers websites/forums. Nevertheless, I'll try to summarise the knowledge and my experience.

Very different, even opposite, problems may arise as decals age.

1. Some high quality decals are fine even after many years. I don't think I tried 30+ yo ones (even though I have a few sets!), but I certainly worked with 20+ yo, and many were just fine. I believe the more modern ones produced after about 2000 will last significantly longer; ~10 yo kits are rather a norm for me. Certainly, the 3 years life stated by Jacob is extremely conservative.

   If the sheet doesn't display any obvious problems (listed below), better checked under magnifying glass, it's worth trying with an unneeded piece.

2. Yellowing/discoloration. Like old paper, decals may yellow with age, especially if exposed to the sun or some chemicals. This should not be confused with yellowing of the whole backing paper, which might be OK. Yellowing of the substrate film is clearly visible as localised background.

   Nothing can be done with this except for replacing decals in one way or another.

3. Cracking. Sometimes, it is so bad that parts of the image fall off or fray. This is very visible and hard to restore. But more often than not, cracks are only visible under close inspection, better with magnification. It looks like craquelure on old paintings.

   This can usually be remedied by spraying or brushing a clear coat on the whole sheet. There are special products sold specifically for this purpose such as Microscale Liquid Decal Film, but most flexible clear coats will do: you just need to experiment with the thickness (number of coats).

   Doing this means that there will be one single decal piece the size of the sheet, and you'll need to cut the parts close to their edges.

4. Drying/lifting. Sometimes the backing glue disintegrates and doesn't stick to the paper anymore (nor will to the model, of course). This is often accompanied with cracking. But if the piece still holds together, it will often be possible to glue it to the model in one way or another, using a clear glue or even a regular clear coat. (By the way, the whole model should normally be clear coated on top of the decals).

5. Sticking. Another glue failure when the decal won't peel off even after a long time in water. Sometimes this can be remedied by adding vinegar to the water, but this will also soften the backing film (which is sometimes even desired, in moderate amounts).

   If the decal has to be salvaged, it's worth reinforcing it with a clear coat (as in (3) above), and then just applying more force to slide them off. Sometimes I lift them with small tweezers or help splitting with a sharp knife: some decals are actually fairly thick and may withstand some "abuse" before tearing off.

All that said, there is a fundamental way to solve this problem: get a new set of decals.

• The decals aftermarket is big and lively. There are many companies entirely focused on it. Many serious modellers are not satisfied with the kit decals and buy aftermarket sets even for new models. There are good chances you can find better decals for your model, even if they are not exactly the same as in the kit.

• Some would not hesitate to buy a new kit only to get the decals sheet from it, and/or a few spare parts. Many kits are produced for decades, and they cost a small fraction of the effort involved in building them.

• Finally, you can print your own decals, as you already discovered. There are a few caveats though, the biggest of them is the lack of white colour (as well as any specialised/textured colours such as silver). Decal paper is sold in two varieties: transparent and white. (And also for different printers: laser and jet). The transparency is the "expected" behaviour, but the printer relies on the white colour of the paper to produce most colours. When applied to any non-white surface, the colours will shift, sometimes dramatically. To avoid this, a white-backed paper can be used, but obviously, it will not have transparent parts and will have to be cut precisely to shape.