Apply primer after cleaning your gutters and leaving them dry for an entire day you can start applying a coat of primer.
How to paint gutters with a brush.
Brush carefully the braces first and then paint them too and leave them dry.
Correct any runs in the paint using the brush.
The undercoat acts as a preparatory coating on the guttering surface increasing the stickiness and durability.
For more see how to clean the outside of gutters.
Use scrub brushes to get any dirt and grime embedded deep within the structure.
If the gutters are not clean before you paint them the paint will not stick.
You will need to use soap and water to get your gutters spotless.