The primer is absolutely necessary otherwise the paint itself might deteriorate the gutter itself.
Cast iron gutter repair paint.
For your cast iron drain pipes i would recommend painting them with red oxide primer first as that will stop rust coming through then dark grey oil based undercoat then your finish black and there is plenty of painting for your wooden soffits me personally i would use a white opaque stain which from my experience protects the wood the longest.
Remove all gubbins dirt flaking paint and rust from the cast iron.
These gutter lining paints are specialist coatings that ensure all surfaces are waterproofed and uv protected preventing leaks for up to 20 years product dependant.
The undercoat acts as a preparatory coating on the guttering surface increasing the stickiness and durability.
It s useful to have a range of tools scrapers wire brushes different grades of abrasives sandpaper not sand it s usually particles of aluminium oxide or whatever abrasives you use.
Gutter repair tape is a thin layer of foil coated with a layer of bituminous adhesive often on both sides of the tape.
The smoother the surface the better the patch will adhere to the surrounding metal.
After cleaning your gutters and leaving them dry for an entire day you can start applying a coat of primer.