Applying decals is just like any other skill. It takes a bit of time, care, experience, practice and patience. And just like any other skill, if you don't practice, accept a few mistakes, and the activity scares the bejeebers out of you, you won't get anywhere with them.
I apply decals all the time, and am in fact going to be decalling several pieces of rolling stock this weekend. This is what works for me:
1) apply a gloss fini***o the model. I generally use Polly Scale paints, so I use a clear gloss coating after the paint is COMPLETELY dry (generally, a week to be safe). I've used all sorts of stuff for this, but I like Testors Glosscoat and Krylon gloss coat the best. Add a light, even coat, and let dry for at least 2 days in a dust-free environment (some sort of plastic box with a lid is best)
2) gather up your tools. I use a metal straightedge, an Xacto knife and plenty of #11 blades, water, decal setting solution (more on that later), a good #0 paintbrush or two, a glass cutting board, reverse-action tweezers (the kind that OPEN when you squeeze them)and lots of paper napkins.
3) Add the decals. This is one of the easier steps, but it requires a few tricks. I use room temperature distilled water to soak decals in (tapwater might discolor the decals). Cut out the decal with a sharp knife (Xacto #11 blade), keeping as close to the lettering as possible. Use a straightedge if necessary, and replace the blade if it tears through the decal paper instead of cutting cleanly (you don't want jagged edges).
Plop the decal in the water (I use a small glass dish), and let it sit in the water for one minute, or until the decal uncurls, or until the decal floats away from the backing paper. You DO NOT want the decal glue to remain on the decal! (it'll leave a white, hazy film under the decal which will NOT go away)
Once the decal is floating around, carefully float the decal back onto the backing paper (with the brush), and grab the paper with the tweezers. Blot off most of the water from the decal onto a paper towel. Using the brush, add a SMALL amount of water onto the model, where you want the decal. Now, position the decal (on the paper) over the general area where you want it. Holding one edge of the decal with the brush, slide the paper out from under the decal. Use the brush (and MAYBE the tip of the tweezers, if you've got the hang of all this and you have a delicate touch) to move the decal to it's exact position. Blot the edges of the decal with the corner of a paper towel, until almost all of the water has wicked away. Smooth out the decal with the brush, and let it dry.
4) Add the FIRST layer of decal setting solution. I use Micro Set for this. Using a fairly broad brush (#0 or #1 works, with soft bristles), quickly add a small amount of solution to the entire decal, and leave. Let dry for 1-3 hours.
5) Slice any air bubbles with a sharp blade, and apply the SECOND layer of decal setting solution. I generally use a stronger solution for the second coat, like Champ's. Let dry 1-3 hours.
6) Check the decals for any remaining air bubbles or ridges, and repeat step 5 until they're gone. Once you're done, let the decals fully dry overnight.
7) add a coat of Dullcoat to the model. I've successfully used Testors Dullcoat and at least two varieties of Krylon flat sprays.
That's it. It might sound like you need three hands at times, but you really don't. Only work on one side of a car at a time, and start with fairly thick (and forgiving) decals like Champ's until you get the hang of it. I generally assembly-line decalling when I can. I can bang out the sides of 10 twin hoppers in a couple of hours.
Modeling the NKP in Peoria, circa 1949
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