Where to get Engineer's blue?

Many thanks to all who replied - even if some did want me a buy a litre of the stuff for 20 quid!

Meanwhile, I tried marker pens - and even car touch-up paint - to little avail, but I think I've now cracked it.

The door in question is a uPVC outside door with a multi-point locking system. In order to lock it, you need to lift the handle as high as possible - which throws various bolts and claw-like devices, which engage with metal plates on the frame. The problem was that the last bit of handle movement was very stiff and graunchy - and I couldn't see which of the bolts was the cuprit.

I finally fixed it by removing the plates from the frame one at a time until I found which one was fouling. Because the door had sagged a bit, one of the claw-like devices was pushing on the bottom of its slot and trying to lift the door. [This didn't scrape any marker off!] By filing out the hole in the frame plate a bit, it is now much better.

why didn't you just lift the door via the adjustable hinges ?

Regards Jeff

Funny you should say that! It's a second-hand door which came with no instructions - and earlier today I worked out how to adjust it in two different planes, using two different-sized Allen keys!

It's now closing and locking perfectly.