One thing I've had some good success with is using a solution of white glue/PVA thinned down with water, a proviso being that the product actually dries clear. Brushed on and ensuring to get plenty underneath the surface, this acts as a softener and setter. Sucks everything down into the panel lines, adheres well to the surface and won't dissolve your decal. It may leave some slight visible residue on the surrounds which can easily be mopped up with water and a cotton bud or such. Then if you don't like the position or something after it's dried, just add water! Finally spray the clear coat of your choice for the final finish. A cheap and practical alternative to the myriad of specialty products. Worth a few tests if you have some spare decals lying around.
