if you are just dipping your brush into the jar and painting the model, then you are dealing with a technique issue.
model paints are designed to be used the same as any other hobby,,,,,,you take paint out of the jar, put it into a depression in a palette, put thinner in a nearby depression,,,,,,mix thinner and paint in a depression between them, until the paint acts the way you want it to when tested
then, to paint the model,,,,you dip the paintbrush into the thinner, stroke the brush on the side of the depression so that the bristles are wet, but not loaded with thinner,,,,,,,,then you move the brush to your thinned paint,,,,,,dip the brush into the paint, loading the paint up into the bristles about 3/4 of the way (Never all the way to the ferrule)
if you feel you have too much paint loaded, stroke some of the paint back into the depression
once you feel you have the brush ready,,,,,,,,lay the paint on the model in smooth, even strokes, painting next to your previous paint line, without overlapping, and Never going back over any strokes,,,,,,,the paint flows off of the brush smoothly, leaving a sort of "stained" thinner behind on the bristles
since you are going to clean the brush after you get done, you can leave that stained thinner there when you get more thinner and paint for the next set of strokes, you only need to clean the brush between colors,,,,,,,or if you have enough brushes, hang the dirty brush in your cleaner, and loading a fresh brush for each color
if you don't linger with the brush,,,,,or "re-stroke" at any time,,,,,,you can paint one color over another dried and cured color with the minimum of bleed through,,,,,,except over White, lol,,,,,,,,that color seems to love getting other colors worked into it,,,,,,for handbrushing over White,,,,,,,let the White cure really well, then brush on some Pledge Future,,,,,let that cure really well, and then carefully brush your trim color over that
If you watch a sign painter (I was taught by one), they do all of these steps in seconds,,,,,,,Load thinner, tap out, load paint, stroke smooth, paint,,,reload,,,,,,etc
I do it so slowly that I just broke down a few decades ago and bought an airbrush, lol
Rex