If you're going to varnish it afterwards which will help to hold anything you've stuck on, I'd have thought you could just use PVA which will dry clear? I've used it to stick beads and sequins on cardboard boxes before and it's worked fine (and you can use it thick to put lots on and make it a bit textured iyswim?) I've never tried varnishing over it though.
I have to admit that I don't have one, so I don't actually know I was just aware of them being used a lot for embellishments, so I presume it's quite strong. I mostly do paper craft stuff which doesn't need such heavy duty adhesive.
Hi, bit new to this craft-that-doesn't-involve-a-small-child malarky!
I bought a couple of cheap and nasty make up boxes - they're made of card, but covered in sticky back plastic. I'm customising them at the moment, and have painted them with acrylic paints. One of them I am going to decorate with fabrics and have bought some mod podge to do it, but the other one I have painted a design on that I want to tart up with gems and sequins and the like. I gather I can only use mod podge if I am sticking porous things? so not sure what to stick the gems on with - also, want to varnish it afterwards - don't know if glossy mod podge would do for that or again if it would be unsuitable because non-porous?