Hi Gary,
Yes, silvering under even 'Aviattic' decal can be a problem and I've experienced this as well.
The white backed decals are not a problem, but I've found application of any of the clear backed decals can cause this silvering.
As you say, an suitably coloured base coat, sanded/polished if necessary then gloss clear coated is necessary.
The surface has to be a smooth and as artifact free as possible.
Also I add PVA adhesive (white glue) to the decal water and stir it until it dissolves into the water.
Richard states this in his decal instruction sheets.
It does help in bonding the decal to the model surface.
Lastly you mentioned using 'Tamiya' X22 Gloss to seal decal edges.
I assume, like me, you've found most standard decal solutions are not very effective on 'Aviattic' decals.
Instead and when necessary, I use 'Tamiya' X20A acrylic thinners, which applied sparingly, will soften and seal the decal into shape.
This is also handy for conforming decal over small, raised detail, such as wing strut attachments, wing foot boards and pre-moulded panels etc.
I pierce through the decal first to release any trapped air or water, then cotton bud to press down the decal and only then apply the thinners.
Beware of using too though, as the decal can 'melt'.
Mike