I'd like to just clarify some things about the "Decal Softners" and Micro Set and Sol, from a scared (and scarred) decal producer's viewpoint.....
There are two types of decal application fluids, one which is a harmless wetting agent and the other, which is a potentially less benign solvent.
The wetting agent is applied to the surface to be decaled and acts to break or lessen the surface tension between the water and the glue solution released from the decal paper and the surface to which the decal is to be applied, allowing it to be more easily moved around and positioned without "premature grabbing" (girls used to accuse me of that in the back row of the Odeon, many years ago...). A very good and cheap wetting agent is dishwashing liquid and a drop or two added to the warm water into which the decals are momentarily immersed works just as well as expensive "magic potions".
Some solvents are, as I said, potentially damaging to your decal's health. Some are really vicious and others more gentle in their action but they all do the same thing, which is to soften or some can even dissolve the ink that has so carefully been laid down to form the decal. Bear in mind the process used to create screen-printed decals involves the serial printing of a number of ink layers, one on top of the other, starting with white and working progressively darker to black. In some cases there can be as many as six or seven colour layers each one carefully aligned with the one beneath, to ensure correct register of the figure.
Dissolving these layers of ink is, as you can see, a risky business - there is no cast-iron guarantee that they are going to settle back in the order they were laid down...... and the more complex the printing and the more layers of ink, the higher the risk. My advice is avoid the use of solvents - at least on our decals! Having said that, we've experimented with the two Micro products (Set and Sol) and they seem to be OK with our decals (all printed by Fantasy Printshop) but use at your own risk and be aware that not all printers use the same ink formulations!
As has already been mentioned, yes, some are acetic acid - nothing else, just that- in solution in distilled water (the words "old", "rope", "money" and "for" come to mind...). Others contain Hydrochloric acid and some seem to be made from Sigourney Weaver's alien's blood, judging from the effect they have!
If you want to avoid silvering, always apply decals to a good gloss surface and if you want the decal to conform to tight curves (such as the edge of a rudder or other flying surface) and/or pronounced underlying detail, then use a hair dryer on low/medium heat played briefly across the decal when in place and then carefully smooth/press the decal down with a (CLEAN and damp) finger or thumb or a damp piece of lint-free soft cloth or paper kitchen towel (not tissue paper or loo paper - it sheds bits of paper everywhere and they stick to the surface of the decal and the model - and especially under the decal, for some reason!).
Another very useful technique is the application of Future/Kleer beneath the decal but be sure to move quickly - it dries fast. Practice first !! I recommend a combination of both these two techniques when dealing with the edges of screen-printed lozenge decals; run a paint brush of Kleer under the edge of the decal, apply the heat and smooth with finger or thumb - removing the excess Kleer in the process - you'll NEVER suffer from decals lifting at the edge or not curling round a wing trailing edge or rudder stripes round the edge of the rudder. PLEASE practice on an old model to build confidence first, but it is a surprisingly simple, satisfying and easily-mastered technique.
