Yeah, this is not a DIY thing.
Like was said, once the fog is there, it isn't going to dissapear completely, there will always be a film unless the pane is removed and cleaned inside & out.
The seal that holds the two panes isn't just a putty, different companies use different things, but it definitely has to be airtight and hold itself together, urethane is often used by DIY Glass companies to rebuild sealed units... but they do not fill the void with argon, so they will fail much sooner and not provide as good of insulation.
Windows should last 10-20+ years before failing, get good windows, get them done professionally, and you won't have to worry. If you try to fix them yourself, I'd bet $ they'll fail again within a year or two :(