Last year's challenge was fraught with varnish problems and it seems this year is absolutely no different. Oddly, this year's problem appears to only be affecting metallic paints, which seems really odd. The armour segments have gone very white, but the rest of the models (and those without any armour at all) are all fine.
Thankfully I noticed it in the first batch of 7, so it's not the worst problem in the world, but it was a unit that had hand painted heraldry on... so that's a bit frustrating. Some google-fu and a daring test on one of the models and it looks like it's fixed *fingers crossed*
A quick coat of hand painted gloss varnish over the top seems to have fixed the metallics. It looks like the varnish was dulling them down so much that the silver just appeared to be white. Now to find a satin varnish I can paint on by hand