cast Iron waste pipes (soil pipes)
have a history of lasting for years often hundreds of years!
a few key points to consider.
1. is it leaking
2. is it rusty
3. is the paint faded & dull
if its leaking a good plumber Ö. Plenty available on Checkatrade can repair the leak
if its Rusty & or the paint is faded & weathered
then lots of preparation to the rust & treat it to prevent future rusting
if the paint is dull, flat or faded by the eliments
Then a wash down with sugar soap wipes & rinse with clean water, allow to fully dry
then lightly sand with very fine Abranet abrasives dust off & paint with your chosen finish.
gloss paint was the go to finish, however Satin & Matt are now also available
my recommendation is choose a trade quality paint.
if your not comfortable doing it yourself (DIY)
choose a Checkatrade member that does exterior painting