The best defense I found was to buy a bottle of DeCal-It, or simular product that refinishes,
restores, and seals decals (Photography and Art supply shops sell a simular product for sealing old negatives.)
Apply it to the decal before putting the decals on the model. If on a budget, then put vinegar in the
water when applying the decals then use the white glue treatment mentioned. I also use distilled water since bleach and other
chemicals in tap water may also interfere with the decals adhesion.
I let my models sit a week or two before applying the final clear to make sure
all the moisture is out. Moisture is the biggest problem when applying a sealant because it will not create a bond to the surface.
The sealant will then "Float" and will have the decal attached to it. When the slightest pressure is applied, then it will crack and
come loose.
If not treated, decals will still deterriorate after sealing them in Future, Varnish, or clear enamel. I have models I didn't treat
that are 15 yrs old and the decals are really comming apart. Planes that have been treated still
look new.
Scott