For thinning you may use the following:
Acylics - Isoprophyl Alcohol mixed with tap water or acrylic thinners available in shops.
Oil Paint - Turpintine (sometimes I use kerosene for oil paints on canvass but maybe not advisable on plastic) or any other oil paint thinners available in the shops.
Enamels & Lacquer Paints - for safety use enamel & lacquer thinners available in the shops (I use industrial lacquer thinners because they are cheaper but I use such with much care as these can eat into the plastic) Caution: Lacquer Thinners are very strong and can eat into plastic and paint layers. Use carefully and in small amounts to avoid sending your precious model into the garbage bin (as what happened to my F-16 once.)
Some water-based artists paints are really just "water colors" that are only good in porous media like paper and may not adhere properly on plastic, so be carefull in choosing the paints.
Happy painting.