Normally I would suggest a good varnish. A varnish will stick to anything. It too will impart a slight amber color to the underlying finishes. You could try a water based polyurethane like <a href="//www.painters-toys.com/varethane-exterior-water-based-finish-semigloss.html" target="_blank">Varethane</a> or a 100% <a href="//www.painters-toys.com/deft-interior-acrylic-wood-finish-semi-gloss.html" target="_blank">clear acrylic urethane</a>.
The water based clears should stick well but using an oil based clear, like varnish, would be better. I know varnish will stick.