i've seen some companies in the states using walnut shells to use as a blasting media as it's the softest thing they used

not sure how they got them to be the fine powdery state they were in when they used them but it might be worth asking for them to use the softest media in order to stop it pock marking the plastics

don't forget that if they are using blasting they may also get hot and warp the plastics too (although you'd expect them to get thigns done right if they know what they are doing, but just something else to look out for, i've only used a sand blaster to clean up alternator and strter motor bodies when i was little )