It is expensive as paint goes, but like Nigel said...it's a tiny thin line so won't require pints of the stuff. Maplin here currently sell a small bottle with 2 or 3 grams of paint in it for £7.99 for example. More than plenty for 20+ repairs of this type.
If the switch current is really really low, and the synth can cope with a fairly high switch resistance, you may even get away with just drawing it on with a good graphite pencil!
Anway...it's food for thought...
rgds