I had excellent luck with using a product called Plastx - about $5 from Autozone/auto parts store of choice.

Removed 95% of the haziness/clouding of the headlights on 4 different vehicles, ranging from 2 '96 C1500 pickups to 2000 Honda Accord. Have not needed it for my 'Stang yet, but I would estimate that it at least doubled the amount of light allowed through. If you get 6-8 inches away, you can see some imperfections, but for $5.00 and a little elbow grease, I was truly amazed. There ended up being no need whatsoever to replace them.

Got the original idea on this forum somewhere. Thanks to the originator and contributor of THAT post!

May not answer your exact question about sealing, but it may be beneficial to you or someone else with a similar issue.

My .02...