I collect old cameras and lemon juice or citric acid applies with a cotton bud is what I use after having seen it recommended on camera collecting forums elsewhere.
Meths and WD40 will both leave a deposit and the latter is frowned upon by camera repairers. Switch cleaner may be ok if it is designed to leave nothing, however some contain lubricants to keep the contacts clean. This can migrate inside the camera. Personally after the citric acid I always spray it onto a cotton bud or use petrol lighter fuel instead. This will normally clean the contacts of chemical, however if there is corrosion or rust this can be removed by gentle application of a glass fibre pencil.
Sometimes you may need to bring a contact up to size and here a small section of the metal covering the cork of a prosecco or cava bottle can work – I haven't tried it with champagne as with model engineering and camera collecting as hobbies I can't afford the good stuff!
Edited By Nick Clarke 3 on 28/12/2018 19:37:01