There are different types of thinners for use with undercoats and top coats. Don't use General Purpose (GP) thinners with your colour coats - it will lead to a dull finish. GP thinners are for use with primers and are a bit cheaper than acrylic thinners. That means you should have two types of thinner before starting the job. Work on the ratio of 2 parts thinners to 1 part paint when buying thinners, so if you buy 2 litres of colour, buy 4 litres of acrylic thinners. The actual ratio of pant to thinners is less than 2:1, it will depend on the type of paint you use, whether you use a retarder with the paint, type of spray gun - the spray-painters on this forum can advise you better than me, I'm just basing this on my own experience with complete resprays over the years.The same goes for the undercoat and GP thinners. buy twice as much thinners as undercoat. Use the GP thinners also to clean out your gun when you finish sraying.