That tells me it IS real turps then Daniel, if it's taking longer to dry. When you use it again, make sure you really stir the paint and turps well before either brushing or spraying, and only do thin coats, leaving time for each to dry thoroughly. It'll be OK when fully dried, and won't affect the decals. It's really a bit 'harsh' for modelling paints, as the latter are of a thinner viscosity and formulation. Turps is really for use with 'domestic' paints, such as used for painting woodwork etc in houses. Although turps will work, in future, if you can't find enamel thinners, get 'White Spirit' often also marked as, or just as, 'Turpentine Substitute'.