Since it appears "bonded", it seems likely that the plastics are similar enough that anything strong enough to soften the crud might also soften the case. So I recommend scraping also,if you remove the bulk of it, some residue will require less solvent and effort to remove.I would advise against using a razor though, even being careful it is easy to gouge the plastic. I recommend a scraper with a nice edge to it made of a softer plastic - or, better yet - wood. It would be difficult to scratch the case with a wood edge. Then once most of the stuff has been removed I would apply rubbing alcohol (90% or so) with a nylon scrubbing pad.