Maxidad66,another good way is to use a liquid called 'Maskol' this is a latex based colour tinted material,you just paint it on up to the line needed,allow it to dry,then paint in the next colour and peel it off when dry.
One of the inherant dangers of tape is that it is just too fierce,to get rid of that excessive tackiness draw it through your fingers first before application,yet another neat trick is to use Dymo ( lettering tape ) tape for etching in a line to paint up to with a craft knife blade,what happens is the slight knife edge cut acts like a reservoir and stops the paint flowing any further.
To experiment with things like this I use a 'Paint hack' this is an old model used for just experimental painting.
Richards thin tape is a good idea also,you could use this idea with a material called 'Chart tape' this is a very narrow masking tape used to generate charts on office walls,you can get this from better stationers
Your dilemma has taxed many a modeller believe me.
