I use Createx......... for T-shirts!
Createx does not adhere to plastic models real well and most of the colors are transparent although many are offered in opaque also. You could use it for single color car models I would suppose as long as you didn't touch the paint at all and put a clear coat over it to protect it.
Mike
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon