How do decals work?

[OldEastRR]

I wasn't sure which forum this would fit so move it if need be.
What I want to know what is the mechanism/process that decals use to stick to surfaces? Yeah I know the water soak, use lots of solvent, wash the surface first stuff but I mean how do the decals work. Why do they stick to surfaces (without using a setting solution)? Do they have a glue back? Do they cling to the microscopic irregularities in any surface? Is their "sticking function" only for painted, unbending surfaces? Why don't they just fall off when they dry?
I've applied a million decals but I never knew how they work.

[mmagliaro]

There is indeed a glue between the film and the backing paper.  When you wet it, the water soaks through the backing paper,
dissolves the glue, and then you slide the decal (including hopefully some of the glue) onto the model.  When the water
dries, the glue rehardens.

If you've ever let a decal loll around too long in the water, come off the backing paper, and just float there,
you will find that it doesn't stick very well because all the glue has washed away.

[Kisatchie]

If you've ever let a decal loll around too long in the water, come off the backing paper, and just float there,
you will find that it doesn't stick very well because all the glue has washed away.

That's where decal setting solutions come into play. Use Microscale Micro Set/Micro Sol, or Walthers Solvaset to "fuse" the decal to GLOSSY painted surfaces. I think the solvent melts the decal into the paint somehow, though I don't know how.


Hmm... I don't know
nuthin about chemistry...

[OldEastRR]

Yeah, they say only glossy paints work but I've done almost nothing but semi-gloss and flat paints for decades with decals and they work fine.
I'm also wondering if the decal solvents from years back were stronger than they are today. I had been using a bottle of Walthers Solva-Set I'd bought probably in 1969 or earlier and it just ran out. Now, using "modern" solvents, I'm finding they don't set the decals as well or do it poorly.

[peteski]

I would recommend reading trough https://www.therailwire.net/forum/index.php?topic=34108.0  Lots of good info on decaling techniques.

I am not really sure how much the adhesive actually aids the decal film adhesion (rather than the film itself just clinging to the surface). Some modelers let the decal images float off the paper then apply them to the model. At that point there isn't much adhesive left on the decal film.

As far as Solvaset goes, it is still very strong. I don't think they've changed the formula.

[nkalanaga]

Some older decals (decades ago) had so much glue you almost had to let them float, or they glue layer would make the decal look like it was glued on.  Those were the ones sometimes referred to as "battleship linoleum" decals.  Modern thin-film decals, on the other hand, don't need to be floated, and will probably either fall apart of fold up if they are.''

I still have a shoebox full of old Walthers decals, and some of them definitely benefit from a good float.  Don't even think about doing it with today's Microscale...

[mmagliaro]

Just to be clear, I didn't mean to imply that I relied on the glue by itself.  Really, the decal just has to stay put well enough after the initial water dries so that it won't move around when I go over it with a quick swipe (and only ONE swipe) of a brush
of Solvaset.  That, and an overspray of Dullcote is what really makes the decal permanent.
 
And yes, Solvaset and other decal setting solutions dissolve the film that the decal is printed on so that
ideally, all you are left with is the ink on the model.  It doesn't completeky make the film go away, but it's darn close
(at least Solvaset is).  That's why if you keep futzing with the decal after you hit it with Solvaset, it will crinkle,
tear, and basically just dissolve into mush requiring you to wash it off and redo it (we've all had that happen a few
times haven't we?     )

[nkalanaga]

The older decals were designed to rely on the glue, because setting solutions hadn't been invented yet.  That's why they needed gloss paint, and to have extra glue, and still often had shiny spots under them.  Modern model decals are intended to be used with setting solutions, and usually won't work right without them.  With the old one, if one has any left, the best idea today is to soak the glue off, then use the setting solution.