I saved the following from the old forum, but I don't remember the author.
"decal application is a fine art. First off store your decals out of light to preserve them, preferably not in a hot area. Next, insects are apparently attracted to the vegetable based glue that some manufacturers use. I have kept decals in excellent condition for years.
Now to application. Always apply to a gloss surface - Never, Never apply to a matt finish or clear film will show and poor adhesion will result. Spray gloss clear before applying if painted finish is not a high gloss.
Use warm water to which a drop of detergent has been added. This aids settling the decal. Do not cut decals until ready to use. Immerse the decal for around 10-15 seconds and then place on a tissue to allow excess water to drain. After around ten seconds check if decal is free to slide from backing. Use a quality decal settling agent. I only use Microscale MicroSet. there are two versions that I use, first is for application over smooth surfaces (mildly reacting fluid) . The second is a more active fluid that assists settling over highly irregular surfaces, like rivet heads. The latter is difficult to use as one is always tempted to keep playing with the decal as it sets, as the fluid causes it to ripple as it dries, eventually smoothing out wrinkle free over the surface. however, in both cases, practice is essential.
With a thin film of decal settling fluid on the area to be overlayed with the decal, carefully position the decal and slide away from backing onto the surface. Apply a little more fluid and carefully remove most of the wrinkles, using a paint brush wetted with the decal fluid. if possible. Otherwise wait until the decal has settled and then gently work out the wrinkles with a paintbrush wetted with the decal settling fluid (wait around ten minutes or so). When happy with result then slowly move the loco, or whatever, to allow the excess fluid to drain away (decal will be positioned vertically). Now await the decal to dry and then carefully using a wet cloth, soaked in clean water, wipe away any telltale signs of decal fluid and decal adhesive residue. When dry, overspray with a matt clear finish. Do not apply heavy wet coat or the spray will reactivate the decal and cause it to ripple. Apply light even coats. The clear overspray that I use to protect decals is a matt clear spray used to 'fix' oil paints from an art or craft store. If I need a gloss clear spray before applying decals, then I overcoat the model with the gloss clear fixative spray, await to dry, then decal and then when dry, matt clear overspray. This will produce a well wearing topcoat for my trains.
the clear spray that I use is fairly inert on paints and decals, as long as not applied as a heavy wet coat. In this case decal will most certainly wrinkle.
The brand that I only use is called 'Crystal Kote', manufactured by Helmar. It's original use is to overcoat oil and acrylic paintings, providing a permanent, non-yellowing finish. I have basically clear overcoated everything that I have built over the past five years with this product and recommend it. Should be readily available in craft and art supply stores. The attraction to me is that I am able to gloss clear spray an item. Await about two hours, then apply decals. Then allow to dry and apply a matt clear finish coat. Await a couple of hours and then handle and assemble the item. All this in basically one afternoon.
I would never attempt a hand applied clear varnish, for as Paul has stated, it takes for ever to harden.
The trick to removing trapped air bubbles is to very gently 'paint' the brush over the decal surface, gently working the trapped bubbles out and persuading the wrinkles to straighten out. I have applied thousands of decals over the years and have had few failures. Basics are gloss surface, a drop of detergent added to warm water to soak your decals and a quality decal setttling agent. The rest is to take care and you will get a professional looking result. Just take your time."