Anyone had any experience getting climbing chalk though customs?
I'm off to Guernsey and I don't think I'll be able to buy chalk there so I'm putting it in my hold luggage. Are customs likely to open it up thinking its something illicit?
Customs may complain, but not likely. I'd make sure to pack it with your climbing gear, along with the chalk packaging. If customs suspect drugs, a simple test will confirm it for them.
However, there's always the chance you'll get someone trying to make a name for himself, and give you a hard time. I had a friend who was stopped by police when he was marking a trail with flour. The cop accused him of throwing handfuls of cocaine on the ground. Nothing ended up coming of it, but it could have.
One other option is to ship it to yourself over there. Just make sure to use a reliable carrier so that it conforms to your travel schedule.
I've carried all my climbing gear when travelling as carry on with no problems in NZ, Australia, UK, and Europe - chalk and full trad rack included. For travel in Canada, I had it all in checked luggage with no issues. Cannot comment on other areas.