Fungus???

[nearthemark]

The downpipe in the pic actually runs inside the utility room, which had been damp due to two leaks.

Yesterday it was painted with moisture resistant bathroom paint, and today these brown spores have appeared. They don't wipe off.

Any ideas?

 

[footprints]

Is it a metal pipe looks like rust spots to me.

 

[nearthemark]

Yes, that section is!

 

[bobasd]

as above - rust spots, possibly from rust thats eaten through the metal pipe? if thats the case then the pipe needs replacing.
what is the pipe - rainwater or soil and vent, and where does it come from and go to?

 

[nearthemark]

Rainwater and waste that's collected via a hopper.

I'm going to route the waste directly into soil pipe.

The rainwater could easily divert into another gutter (or even via stack into soil).

 

[bobasd]

your soil stack will not be discharging into the soil -
soil stacks drain to manholes and then the main sewage line - or go to a tank.
dont connect the rainwater discharge to any soil stack unless the alternatives are not feasible.

 

[nearthemark]

Sorry I wasn't clear, I am aware of where the soil pipe goes!

 

[footprints]

Scrape loose rust off and give two coats of Zinsser BIN or similar rust blocker before repainting.