If the gutters are not clean before you paint them the paint will not stick.
How to paint gutters with a brush.
For more see how to clean the outside of gutters.
Apply a thin layer of top coat with a paintbrush or a sprayer.
You can also apply a thin layer of top coat with a paint sprayer if you d prefer.
Use a small amount of top coat on the paintbrush so it spreads thinly on the gutters.
Apply primer after cleaning your gutters and leaving them dry for an entire day you can start applying a coat of primer.
Brush carefully the braces first and then paint them too and leave them dry.
Correct any runs in the paint using the brush.