Thanks - Yes, it will need to be calibrated. Price doesn't look too bad - i was offered a Sealy one for 250 quid but it looks to be made primarily for car paint testing
@Welderpaul I bought a cheap eBay one some years ago for checking paint prior to setting about the car when detailing it. It won't read multiple coatings or off ali etc, but works OK on steel bodywork. Came with shims for calibrating and did/does what I need.
They're probably cheaper now than when I bought it, iirc £100[ish] then.