In some case when I paint small areas the paint seems to go on the borders instead spread in the water.
I usually notice this effect after watercolor dries.
Should I consider these outlines mistakes?
Maybe I'm using a brush that is too big.
I usually paint on A5 paper and small subjects, like flowers, with a n.6 Kolinsky Sable brush and some times it hold too much water.
You get these outlines when the pigment collects on the edges of the water you have laid down. This happens when you use a lot of water, causing the pigment to float around in the "puddle" and gravitate towards the edges as it dries (if you're interested in the physics, there's even a name for it: the coffee ring effect!).
To avoid these outlines, you need to use less water when you lay down the color. You should have enough water to wet the entire area, but not so much that it puddles on the paper. You may find that you need to dilute your pigment more - before the wash - to achieve the same color. (Note that diluting pigment is not the same as using lots of water to apply it!)
This will involve some amount of experimenting to get a "feel" for how much water is necessary. Also, consider using a smaller brush since you mention yours sometimes holds too much water. I would recommend trying different ratios on a scrap piece of paper to see what works best with your pigments.
Consider them options. If you like them, leave them ( or part of them) &, if not, moisten brush & buff them out.
