model making by scaling is pretty hard. That in mind, a few pointers that can really go a long way: remember to thin your paints using water, makes it easier for colour gradients and gets rid of the awfully plasticy and rough look of dried paint. Thinned paints also make it easier to maintain detailed textures AND also helps to eliminate the visible brush strokes. Also remember to use fine tip brushes. Regarding painting lines, Use tape, much like the way internal designers use painter's tape so you can create a nice smooth and crisp line. Same with clear peices, put tape over them so they dont get dirtied
And most importantly, DONT RUSH IT. model making inherently requires patience due to attention to small details, creating nice crisp lines and drying times for adhesives and paint layers