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Its posted but it doesn't come up under Rust ? http://roof-cleaning-institute.activeboard.com/index.spark?aBID=123190&p=3&topicID=27476833 I myself had not had 100% results with rust stains i've used 3 types and tried even full strength , the same goes for soot stains from oil burners i find these stains on the prefab chimney, not the brick style,must be the style of the cap i also find that this style chimney has rust stains on the shingles either from the stainless starting to rust or the sheet metal screws they used to put the sections together or both

-- Edited by Roof Cleaning Long Island New York (631) 220 6985 on Sunday 30th of August 2009 10:07:19 AM

-- Edited by Roof Cleaning Long Island New York (631) 220 6985 on Sunday 30th of August 2009 10:10:45 AM

Thanks Mike , do you dilute it with water or just 85/15 on the mix and do you brush, pump spray ? and how long do you leave it on also where is phosphoric acid sold wow thats a lot of questions lol

Try oxalic acid, or Rust Aid from home depot. 

Roof Cleaning Long Island New York (631) 220 6985 wrote:

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Thanks Mike , do you dilute it with water or just 85/15 on the mix and do you brush, pump spray ? and how long do you leave it on also where is phosphoric acid sold wow thats a lot of questions lol

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For roof cleaning I cut it down with a little water to make it spray better using my pump sprayer.

85% is the strongest phosphoric acid you can get.  Univar or Brenntag sells it.

Adding a little muratic acid to it really helps.

For vinyl siding and concrete I just use a bucket and a brush.

I may or may not cut it down with water depending on how bad the rust stains are.

Like Brian says Oxalic acid also works, Larry in Florida uses sulfuric acid.

use a hydrofluoric based product for rust

Yes, i believe the HF does the work but they put in Sulfuric because of the smell.  The HF i use does not really have a scent so you can get in real trouble with it if you dont know what you are doing.  Kind of like how natural gas doesn't smell but they add rotten egg smell so you know when there is a leak.  HF does not really burn the skin but soaks in and kicks your arse later after its too late.  Pretty insidious stuff, but boy does it work on rust.  Faster than love on algae, im talking right NOW.  Of course, keep everything wet and rinse well, it smokes greenery super easily.

I know a guy that had a five gallon bucket of HF in his work van. He left the cap off the pour spout and ended up with a puddle of HF on top of the five gallon bucket as he drove around for a week. Doesn't sound so bad right? Well he proceeded to SIT DOWN on said bucket to work on some equipment in the van. A few days later he wasn't sitting at all and couldn't for about 3 weeks