The following is a procedure for applying decals that I learned from an FSM article I read a long time ago and have applied to my decaling technique ever since. I have improved on it slightly over the years and I hope you learn from it some valuable tips.
List of Tools Needed:
Soft rag or bathroom tissue
Setting Solutions (See below)
Soft 1/4" wide brush
Cup of lukewarm water
Non porous surface (Piece of glass or plastic)
These liquids are an absolute necessity in applying decals to all surfaces. There are basically two types: setting and solvent. The different is the strength and its affect on the decal itself. The setting types are the weakest and the solvent types are the strongest. I personally use three types. In order of strength they are Microscale Microset, Microscale Microsol, and Walthers Solvaset.
I use Microset on all decals on all surfaces. I use Microsol on slightly irregular surfaces or to have the decal conform to minor surface detail. I use Solvaset on the most irregular of surfaces or on areas of great surface detail. I have had decals conform to the most irregular of surfaces like Zimmerit and louvres etc.
It is essential that decals be applied to a glossy surface in all cases. If the surface is eventually to be flat, you must apply a flat clear finish over the dry decals to achieve this. If you try to apply decals to a flat surface, you will see a "silvery" effect on the clear edges of the decals as they will not blend into the surrounding area properly. This is not a problem with race cars and such as the surface paint is normally glossy in the first place. On military models such as planes and armor, I first apply a gloss clear coat over the flat finish first. I normally use Testor's Glosscote, apply the decals, then spray on Testor's Dullcote. When done correctly the decals look like they are painted on. Check out the military models in the galleriies to see this for yourself. This double clearcoating also has the benefit of allowing you to apply the initial weathering to armor prior to flat clear coat and also even outs the finish on camoflage finishes.
I normally cut out the decals not too close to the outline as most decal makers taper the clear decal film slightly away from the outline. This tapered edge allows the decal to "blend" into the surrounding area better. Of course some decal makers such as Cady cover the entire sheet with decal film as part of their process and you then need to cut the decal close to the outline in order to minimize the clear edge.
I next dip the decal in the water for no more than 15 seconds and then place it on the non-porous surrface and wait for it to loosen from the paper backing. This is by far the most important tip I learned from the FSM article. Decals come with a water soluable adhesive on the back of the decal ink and carrier film. It is this adhesive that holds the decal in place on the model. When you dip the decal in water, the water dissolves the adhesive and allows the decal to slide off the backing paper. Many modelers (including myself) make the mistake of leaving the decal in the water too long or place the wet decal on an absorbant surface. Doing either of these things just causes too much of this adhesive to dissipate into the water or the porous surface. This can cause the decal to curl up at the edges or not stick at all. You can fix this as I outline later on, but it is best to prevent it at the beginning. By just dipping the decal for a short time into the water and placing it on the glass or plastic, you maximize the amount of adhesive present and insures a good bond.
While I am waiting for the decal to loosen, I apply some of the Microset solution to the surface where the decal is to go. This does two things. It allows the decal to remain mobile durign final placement and insures a good bond to the surface by allowing the decal to conform to the minor surface imperfections.
When the decal has loosened, I place the decal with the backing right next to the final location and using a damp finger I slide the decal onto the surface and slide the backing paper away while letting the decal "fall" to the surface. With practice the decal will be very close to the final location and will be "floating" on a film of water and the setting solution. I use the tweezers to carefully move the decal into the final position. I then use the soft cloth or bathroom tissue or Q-tips to carefuly blot up the excess moisture and press down the decal to the surface being careful to start at one cormer and working over the entire surface of the decal. Try to avoid presing down on the entire decal at once as you could accidently lift up the whole decal. I then apply a thin film of Microset over the decal carefully blotting up any excess. If the surface is flat or the decal does not have to conform to any more surface details or irregularities, I let it dry as is.
If the decal has to conform to any surface details or to a compound curved surface you will need to go to the stronger stuff to soften the decal to allow it to "snuggle" down. The type of solution to use comes with experience, but I always try the weaker stuff like Microsol first before going to the ultimate Solvaset. Microsol works with most compound curves and most surface details like Zimmerit and recessed panel lines etc. High details like hinges, hood pins, or really big compound curves require the use of Solvaset. Appication of the two solutions is different because of the difference in strengths.
I apply Microsol with a wide brush after I have placed the decal and blotted out the excess setting solution and water outlined above. I let the MicroSol work a bit to soften the decal first. I then use the Q-tips or the tip of the soft rag or paper towel to GENTLY press down on the decal to begin the process of conforming the decal to the surface. I sometimes apply a little more Microsol with a Q-tip or brush to help the process along. In most cases if the surface has some Microsol on it, time and gravity will let the decal snuggle down. I normally monito the process over the next few minutes and use the Q-tip or brush to snuggle it down.
On those really stubborn areas I use Solvaset. CAUTION IS IMPORTANT HERE. Solvaset is very strong and works fast. Solvaset was developed primarily for the railroad guys to apply decals to the sides of railroad box cars that have a lot of surface detail that required the decal to look as if it were painted on. Next time you are in the hobby shop check out some cars to see what I mean. Normally I just apply the Solvaset over the area and just let it do its thing. The decal will sometimes wrinkle and look like hell, but it will eventually fix itself. You can "nudge" the decal to conform to really tuff areas with a Q-tip dipped in Solvaset, but this takes practice. The nice thing about Solvaset is that if it does not work totally the first time you can apply more after the decal has dried.
Bubbles I Got Bubbles!!!
Sometimes you will get trapped air that will cause a bubble or wrinkle in the decal when it is dry. This is easily fixed by carefully slicing into the bubble with an X-acto knife (new) and then applying a little Solvaset or Microsol and pressing down with the Q-tip. Presto bubble and wrinke gone.
Sometines you get a kit with old decals in them. Before you try to apply them you will need to pretreat them. Three things can happen to decals with age. Yellowing, degradation in the decal film, and degradation in the decal adhesive. If the decal has yellowed, I just leave them in the sun on a window silll for a day which in most cases will bleach out the decal. Next I apply one or two coats of Microscale Clear Decal Film to the entire sheet. This stuff is great at preserving old decals and keeps them from cracking upon application. I use a wide soft brush and lay it on in even strokes and let it dry before applying another coat. I normally let it dry overnight before attempting to apply. Once you do this you will need to cut the decal close to the outline as the clear coat is now a continuous surface. Finally sometimes on really old decals the adhesive is so old that it will not stick to the surface of the model. In this instance, you need to replace the adhesive. This is done by making a dilutted solution of white glue and water (75% glue 25% water) I brush this on just before and sometimes after application of the decal. After the decal has set a bit you can then apply any and all ot the setting solution to allow the decal to nuggle down. It will look milky at first, but it will dry clear.
Check out the Devastator model in the planes gallery. This kit was over 20 years old when I got it off e-bay and the decals had lost their adhesive potency. I used all the above methods and was able to have the decals conform to the corrugated wing surfaces with no problem. The decals look as if they are painted on.
I have had to sometimes sand off old decals on models that I have refinished because this technique works so well. If you follow the above steps you will be asked by someone sometime, WOW! How did you paint those markings on?