Why not indeed. You can do great things with self-printed waterslides, although for sure there are aspects which can be finicky and require care.
First the brand: there are many that work fine; everyone has their favorites. I've been using the Sunnyscopa
paper available on Amazon and it has been 100% satisfactory for me, while reasonably priced.
The inkjet dyes to not print completely opaque on the decal film. Therefore, the darker the color of the background paint, the less the decal printing will show up.
On a light colored background paint (especially white), clear-backed film works best. You can leave as much border around your design as desired, and the clear film will disappear. If your design is very dark or black (like lettering), it'll work on almost any background color that isn't too dark. Designs that rely on a lot of color, though, will start to suffer more quickly as the background gets darker. Therefore, at some point it becomes worth it to use white-backed decal paper. The trick, though, is that you'll need to include the background color in your design, and it can be hard to get an accurate match to any color other than black.
There are other strategies to make this work. Sometimes you can do double-layer decals: lay down plain white paper (unprinted) first, and then a a clear-backed decal on top. All this needs to be fairly carefully planned out. If you're trying to do something like print white text on a purple background, it's going to be really difficult getting the background color matched. In many instances you will need to plan your paint scheme around what can be accomplished with the decals.
Thanks Neil, really appreciate all the info and advice.
Inkjet waterslides absolutely need to be clear coated. I use Krylon UV-resistant gloss clear. There are many others that work. In my experience, even fully clear-coated decals can bleed a bit of ink at the cut edges when soaking, and so when printing something like a black background on white-backed paper, I'll make the decals a bit oversized, and then trim the edges *after soaking* and right before application. Sometimes I'll use laser decals for these, since they don't require clear-coating and don't run in the water soak.
As with many things, printing your own waterslides is something that may take a few iterations before you get all your techniques fully dialed in. But the results are worth it.
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(disclaimer: there are a few laser decals mixed in there, but the big color stuff is all inkjet)