John Halfpenny – your technique would obviously work but I'm not sure why you would not put the lid on after using the paint and see if the contents skin over, rather than deliberately allowing skin to form.
NDY – Inverting the can briefly to let paint seal the lid joint is good practice. I should have mentioned this in the first place
Martin King – The solvent used needs to be the same as the solvent used in the paint formula e.g. use the thinners recommended for that product. Air-dry enamels are very likely to be compatible with white spirits but spraying "enamels" will need toluene or just as good is xylene. The recommended thinners for spraying enamels will be compatible with air-dry paints but you need to be cautious to use as little as possible, or you will cut the viscosity to that of water.
In this day and age you might find it difficult to buy Methy Ethyl Ketoxime but then again there are some pretty dangerous chemicals sold on ebay that should never be allowed to get into the hands of unqualified people