A few passes over a strong magnet sufficient or should a chemical solvent be used?
Either should work...I'll bet that scratching the shit off with his keys worked too ;) I'll have to remember that when it's time to get mine done.
My suggestion is not to alter the card such that a visual inspection would show. In most states it's a felony to modify or alter a drivers license, they are considered the property of the state and not yours personaly.
Next time a cop asks for ID and you hand him your d.l. you could be in for a lot more than couple hundred dollars and a ticket.
"Yeah, officer, I dropped my ID in the street and it got run over. Don't give me such a hard time about it, I'm just glad I got it back..." I really doubt you are going to get convicted for accidently dropping your wallet because your ID got scraped up a little bit on the back side. Seriously, the best way is a strong magnet. You can also put it in your shirt pocket and iron it, then apply a strong magnet. The heat will make it lose its magnetism more quickly; just make shure you don't melt it. :)