Much good information on decals has been posted here and in the main forum. A search should turn up lots and lots of good posts. In addition to that, there are a few other things to keep in mind
1. I have had excellent results applying decals over flat paint, including the ultra flat auto primer rattle cans. This is contrary to the conventional wisdom that calls for a gloss coat UNDER decals. Most model railroad paints are flat and many folk recommend spraying a clear gloss coat over the flat paint and applying the decals over the gloss. I don't bother, I just decal onto flat paint and it works.
2. Trim the decals as close as possible to remove as much decal film as possible.
3. Soak the decals in water until the decal slides easily over the paper backing. Too short a soak and the decals tear from glue that isn't wet enough. Too long a soak and the decal adhesive soaks off and the film gets soft and tears.
4. Place the wetted decal still on the backing paper on the model as close where it wants to be as possible. Then slide the decal off the backing paper onto the model or slide the backing paper out from under the decal. Decals much prefer to stick to your fingers, paper towels, blotters, tweezers, anything except the model. They float on water and drift around. You may have to blot up excess water before the decal settles down. If you touch the decal with the blotter, the decal will invariably stick to the blotter.
5. Use decal setting solution. Solvaset has worked for me for many years. Wait till the decal is settled in place and any wrinkles or air bubbles have been worked out. Don't add so much setting solution as to float the decal off the surface. Setting solution softens the decal film. This means you don't want to move a decal AFTER applying setting solution.
6. Let the decal get good and dry, at least over night. Then give the model a shot of DullCote to make the decal film invisible. Once many years ago, I used some other clear satin spray instead of genuine DullCote. The substitute coating made the decals crinkle up. Since then I have used nothing but genuine DullCote.
David Starr www.newsnorthwoods.blogspot.com
Unless you want effect of new signs, always decal first and then weather. Decaling a building is almost the same as decaling any other type of model. Much depends on the decal quality, but overall the techniques are basic. Prepare the surface of the building by making sure it is clean and dry (make sure no dust or humidity is over the structure), then applay a layer of transparent glossy paint, or Future, and let it dry. Cut the decal off the decal sheet. Dip it in water for no more than 10 sec. Let it stand (wet) for about 15 sec, or until the decal slides off the paper. Take care when sliding the decal off the paper, some decals may take longer for the glue to liquify, applaying to much force to the decal may brake it. Apply a small amount of Decal Set to the surface with a brush. Place the decal (still attached to the paper base) next to the place where you want it to be. With the brush or your finger slide very carefully the decal off the paper to the model. Use your fingers or a brush to gently move the decal until it is in the correct position and place. Try not to move it too much or else the glue will stain your model and will be lost. After the decal is in place, use a piece of paper towel to dry the excess water. Apply a bit more of decal set or, if the surface is very irregular, a bit of decal solvent (micro sol) to thin the decal and make it fallow the contours of the surface. Let it set over night. When all decals are set, applay a coat of transparent matte paint like Dull Coat if it is a masonry or concrete facade, for metalic or PVC siding facades you may use a coat of Future to give it a semi gloss finish.
You could replace the decal setting solution with white vinegar mixed with a bit of isopropanol. For very thick decals always use a decal solvent like micro sol. If you're planing to use enamels or laquers for weathering or finishing over the decals, protect the decal with a layer of Future, otherwise the solvents of those paints may attack the decal and destroy it.
Happy modeling!
Depending on what you did for weathering, that may have been a mistake to do that before decalling for a couple of reasons: 1) Some forms of weathering leave a rough surface that will make it hard to get a smooth decal job and 2) Usually on the protptype, the logos and lettering are done right after the paint job, so these things, usually from decal sets on our models, will also be weathered. Proper sequence should be paint, decal, weather.
What are commonly called "water slide decals" will adhere best to a smooth or glossy surface. I usually use distilled water for my decal work, and change it out often if I'm doing a lot of decals. I have hard water and there's still a few locos I need to redo that have an "aura" around the decal where the water dried and left a noticible mineral ring around the decal. I also make sure that while I'm doing the decal work, the surfac I working on is horizontal, to help prevent sliding or running during the next step. Once the decal is in place, apply the decal setting solution that is recommended for your brand of decals. Don't touch this until it dries, especially when it wrinkles. After all sides are decalled and dry, do your weathering and apply a clear flat spray to seal everything.
Could someone go over the right procedure for adding decals to a plastic model kit? I have a couple building built, painted, and weathered, now I want to add the decals. What is the preferred method? Thanks for the help.