At college we were taught to use tippex to paint onto parts we didn't want the solder to flow onto. This works fine, but the smell of burning tippex is awful, and I don't think it is very healthy to breathe in.
Red rouge powder, mixed with a little water - does the same job, quicker and easier.
You can buy it here (UK). A little goes a long way! That pot should last for years!
I mix a tiny amount with water, and apply with a small artist brush anywhere I don't want the solder to flow onto.
|Red Rouge powder|
|Chain link to be soldered|
|After a bit of cleaning with a silicon wheel - almost like new!|