my hasegawa P3 had to re visit the paint shop due to yellowing of the white paint
which was water based Humbrol paint which was not good to begin with.
The entire kit had to be stripped and re painted - bye- bye roadrunner!
Keep your kits out of strong sunlight seems to be the only answer to this.
I have a box full of old decal sheets dating back to the early 80's most of which are in good condition.
Keep 'em cool and dark , no problem in this climate,dry - now thats a different matter !
I recently bought a Hase F4J and the decals were slightly yellowed,so I cut off the offending yellow bits,which also eliminates silvering.
I dont know about theUV barrier qualities of different varnishes but have never had any problems with Gunze or Tamiya acrylics and if the whole thing goes bad a few years down the line some isopropyl alcohol will strip the offending paint a
la P3 mentioned above.