Yes a spray varnish, although not great is really the only thing that can be used now....however most of the inks used in "domestic pens" unless black will fade in direct sunlight very quickly anyway......so I am afraid your piece is not going to last 500 years!!!!

Unfortunately this is a bit of a catch 22 situation. If you add a fabric fixative to the block ink and get it on thick enough to cover, it will be too thick and will peel off and crack, alternatively using a screen ink will tend to be a little thin and the colour will not always cover.

So firstly don't add fabric fixative to the block ink!, the block ink is just not flexible enough to do the job. The answer is to work with the screen inks, choosing light colour fabrics and "inking" up well......it is possible to thicken the screen/fabric inks, however as described above, if they go on really thick (which means they cover really well) they will crack....so the balancing act is to get just enough on, without it being to much to crack......sorry I can't be much more help than that! Good luck with it. (another way to get basic prints done is to use a sponge with the design cut out of it, or then to move to screen printing.....but no that isn't block printing anymore!!! :)