A clear varnish should help protect them, but not sure if there is any recommendation for the adhesive transfers?
I've just fitted a waterslide 'piled arms' to my rear number plate bracket and it's had two coats of brushing varnish over the top. My only mistake was rubbing the gloss back for a key before applying the waterslide - I only did it with a scotchbrite enough for the first varnish as I thought it would be easier before the transfer went on, but the background to the transfer looks slightly grey now not a deep black - oh well I'll know next time (I don't think I've applied one since I did airfix kits as a kid!)
