Depends on how deep the scratch is. If you google "LCD scratch repair" you'll see there's a ton of techniques and products, BUT every one has the possibility of ruining the screen... Not worth it in my opinion, not to mention what a lot of sites wont tell you is that if you do use a rubbing compound (or toothpaste), it will sand away at the matte finish of the screen giving you a horribly reflective glossy spot on the screen.
Hmmm.... Maybe a small amount of vaseline on the scratch, then apply a screen protector to make it seem flawless?... (haven't tried it, just a thought)
There's really no good way to polish out a scratch from a matte surface screen. The plastic would be shiny--but it has had a texture added to it, and the scratch has removed that texture. Anything you can do is going to either remove more of that texture--or create a new one. Odds are, it will just leave a bigger shiny area and make the damage worse.
I've done a lot of refinishing (media blasting, wire brush pecking, polishing, etc.) and wouldn't risk doing more damage to my Nuvi, I just try to keep it in a piece of cloth or an old sock to prevent more damage. Ignore it for another year or two, they'll find some compelling reason for an upgrade to a new model anyhow. (Like Magellan's new 7" screen models.)
in my opinion is not worth polish, the type of screens of the GPS are not very "strong", I belive that the screen would stay "white", giving that impression of "sun eat", but I heard about one product that is like one glue that you clean softly the screen, than put that glue and clean ( the glue will fill in the scratch's ), but I do not belive that would be nice :s because with the time it would stay the same perhaps.
Garmin has improved their LCDs a lot and the latest models like the 14xx series all have great scratch resistant LCDs. I use the 1460 and it's scratch resistant. Been using it for months and so far still no scratches. Hope all who are considering which model to buy will find this tip helpful.
Try a substance called plastX. Great results have been achieved by using this. Remember to use it very sparingly so as not to damage the touchscreen. Also I would say try it on a small area before coating the entire device. Egg whites are also known to provide a solution but it sounds a bit dodgy to me.
It is best to put a screen protector on it when it is new. They are cheap ($1 to $2 each). They can be replaced if it is ever scratched. Much easier to do than trying to remove a scratch on the actual screen.
Just polished the screen of my 800 bike computer with meguiars mirror glaze fine cut cleaner . patience ...2 days of periodic work 1 finger and an old tshirt . took out on 2 rides in varied lighting .. very pleased .. give it a go ! Robert M