Hi Gang,
I don't know if this will help anything, but recently I had occasion to attempt to build a bonus kit I got from a guy that sent it along because I bought 10 1/35 kits from him on EBay. Well I noticed the box art is an exact knock off or the Tamiya kit in 1/35th. This kit however is 1/72 scale, it is an M60A1 W/ reactive armor. Well to my chagrin, upon opening the box, there was no reactive armor and no decals. I wanted to try a 1/72 model so I decided it can be an M60A1(plain wrap), but no decals!? In 1/72 I am hard pressed to paint them on sooooo...... ! I own a Kodak all in one 5500 printer. I worked for Lockheed as an engineer for 23 years and worked for 6 years in the Calcomp div. designing printers / plotters. I took the decal sheet out of my Tamiya 1/35 kit and placed it on two sheets of white paper ( 94 lumens) and 1/72 is approx.. 1/2 the 1/35 scale, so I set it to reduce by 50% and printed them on semi-gloss photo paper, ( I don't have any decal paper at the house yet), well they came out perfect! I brought them into photo shop and removed the light blue color copied ( because of the light blue carrier paper the decals were mounted on originally) and then reprinted again on photo paper and viola an exact match in approx.. 1/72 scale! I can't speak to the decal paper yet, but I do know one thing. Printers like an HP use a dye based ink with an isopar type carrier ( isopar being a long chain polymer, such as kerosene for example) or even an alcohol or water based carrier to make the ink slurry that is deposited on the paper when the ink is boiled by the print head causing a bubble (or dot) to be deposited. A more severe problem this type of ink exhibits is it is basically a dye and therefore is very much influenced bye the substrate it is sprayed on, i.e. will it be absorbed or will it float as it would on say a piece of glass or even a lightly plastic coated (I'm guessing on this part) piece of decal printing paper. With a dye based ink subsequent layers are actually partly overlaying and partly dyeing the dye already laid down and as we know overlaying of the basic colors is what creates all of the different shades and hues we all know and love.
The reason I own a Kodak copier is because it uses a pigment based ink. No matter what it is deposited on pigment is pigment it is made from pulverized minerals, not crushed berries and the like. In general they will not stain each other, they overlay very evenly and the opacity is for the most part a non-problem, such as blue dye looking aqua, or the red looking orange. Most of this is caused by substrate characteristics, such as paper quality, absorption rate, whiteness, coatings, etc. etc. For instance many papers have a certain amount of paraffin wax in them to help hold the fibers together. Which side of the paper you actually print on makes a difference, most packages of descent paper actually have an arrow or such telling you what side to print on ( its the smooth side) there is a smooth side and a wire side. The wire side is the side that wire brushes of a sort contact to move the newly created yet still wet paper along in the roller conveyor machine and the smooth side is the side that is against the smooth rollers during this process. You can test this by placing a piece of paper in some water it will curl up. It curls toward the smooth side as the wire side is more porous and absorbs more water and expands like a sponge forcing the paper to curl inward towards the smooth side.
My whole reason for chiming in is to say that working with dye based inks and papers (especially non absorbing decal types) will almost always be the cause of unpredictable results, unless you control the paper (always exactly the same, date and lot etc.) temp., humidity, even the dye lot of the ink used in the cartridges will have an effect. However you will always eliminate at least 50% of the problems with a pigment based ink and therefore yield a more repeatable result. I would recommend that if you need a certain decal and may need it more that once. Once you do yield good results make a bunch of copies for future use and try a pigment based printer/ ink. I assure you this will eliminate a lot of headaches! I any of you would like to know more I would be glad to discuss it there are wax based layered type inks, thermal types and many different type papers. My father was a printer/ lithographer for all his life and later sold inks to major printers, I guess you could say that if you cut me I'd bleed ink! Hope some of this helps! Let me know! Sorry for the long winded speech.
Your Fellow Modeler,
Newarmorman