If you had trouble with the humbrol bleeding or running away it's most likely that you've either diluted it too much or your compressor was set to too much PSI.
In regard to adherence it's important to not just use primer on plastic parts but something that forms a bond between the plastic and the primer. In German it is called 'Kunststoffhaftvermittler', lol. If you don't use it in many cases the primer won't stick. Also as word to the wise always stay within one type of paint (eg, only acrylics, enamels or lacquers). If you mix them you can get all kind of unwanted effects.
Hope that helps 