I have written some months ago a section
on my homepage about decals; maybe this might be useful for somebody too.
I made some experiences in the past with inkjet sheets (transparent ones), but wasn't satisfied. The colors were not covering like I wished to do and one of the main drawbacks, as already mentioned, is that you cannot print white. So wanting to use white you just have to use the transparent one on white objects (what's not always possible) or using the white sheet; the later one has the drawback that you have to cut out it very carefully to have no white edges -- and to be honest: in case using white decal sheets you also can use white regular sticker sheets (which probably might be much cheaper and easier in handling).
Using inkjet sheets you have, as already mentioned, to use some spray for the colors not to run when put into the water. For this,
Richard you can also use simple clear varnish, either by brush or by spray/airbrush.
I personally prefer ordering my personally designed decals at a printer, found at the Web (cf. link above). He has the possibility to make printouts in much higher resolution than I know from any laser/inkjet printer and even can print white, gold, and silver.
Some samples for these special decals are the