Hmm, finally took courage in my hands, wet my finger and rubbed the pattern hard - ever so slight fading of the colour and transfer of pigment to my finger... hmmmm.... not bad, but I may go over the design with clear nail polish to be sure. Was thinking PVA glue or polyurethane spray varnish, but they go cloudy and whitish when they get wet.
I think a matte top layer nail polish should work.