# POR15 Metal Prep - Phosphoric Acid by another name?



## RichardG (10 Aug 2021)

I've used POR15 metal prep quite successfully before painting both new and old metal and need to buy some more. If you haven't come across it before Por15 metal prep.

_It is the ultimate in the relentless fight against rust. It not only dissolves away the rust it also etch primes the clean surface and leaves a Zinc Phosphate coating to prevent further rusting._​
_This amazing liquid is non-toxic, non-flammable and non-corrosive. It ensures perfect paint adhesion and gives better welding conductivity._​
However, I wondered if Phosphoric Acid would do a similar job but at a fraction of the price? Anyone else have any experience of this or other metal prep treatments?


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## gcusick (10 Aug 2021)

I think there are at least 2 important differences between POR15 and phosphoric acid - POR15 is said to be non-toxic and non-corrosive. Phosphoric acid, on the other hand…


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## gcusick (10 Aug 2021)

However, the Material Safety Data Sheet for POR15 tells a slightly different story



https://sds.por15.com/Metal%20Prep/POR-15-Metal-Prep-40201-40204-40205-40216-40255-240201-240204-240205-240216-240255-SDS.pdf



Key ingredients in POR15 appear to be zinc phosphate and phosphoric acid.


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## RichardG (10 Aug 2021)

Great find, so as you say it's basically 10-12% phosphoric acid with the key ingredient 2-5% zinc phosphate. I wonder if the ranges protect their exact formula or just aren't critical?


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## BillK (11 Aug 2021)

Pure phosphoric acid leaves a thickish blackish end result on rust. I think Jenolite is mostly phosphoric acid. It also leaves like a glossy semi-tacky coating on bare non-rusted steel that wants taking off before paint. Pretty good for small areas you can't soak, but you have to keep it wet or it stops working.

Citric acid is dirt cheap (like a kilo of crystals for a tenner delivered) and safe, it seems to sort of etch bare steel slightly leaving a light greyish dusty coat, which brushes off easily. If you can dunk a thing, using hot water + citric, a plastic container with a bit of scrap ply as a makeshift lid, gets it done quickly. Wash & degrease the bits first. I'm about to do this again with a few sheet steel things.
You don't have to be fussy over the mixing, especially if doing the hot water thing, it works much faster anyway then.

One product out there, can't remember which, is basically citric acid. There's a gel variant of it so it can be glopped onto small areas that can't be dunked. I've used that one, it works but is messy.

Citric dunking's also handy for getting zinc plating off things, want to do that outdoors because of the gas it gives off. Going to do this too, for a drum sander build, a dust port/manifold is galvanised and I need to weld it to a steel cover.


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## RichardG (11 Aug 2021)

Thanks for the details on the other options, I think I'm going to stick with the Por-15. It may seem expensive but I love the way it removes surface rust and leaves a zinc coating all ready for painting. I've also found it acts as an good etch primer for clean steel so multi-purpose. The other options look much better for heavy rusted surfaces though...


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## TFrench (11 Aug 2021)

Por 15 is definitely the best rust treatment I've used, because of the coating it leaves. Evaporust is decent, but these days I use citric acid. Does the same thing but cheaper. Only thing I have find is that leaving things in too long can leave a crystallized deposit that's as hard as the rust to remove.


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## Awac (11 Aug 2021)

Phosphoric Acid gets my vote, just dilute it, drop whatever in, walk away and let it do its stuff for a fraction of cost, reuse for quite some time as well.


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## Dibs-h (12 Aug 2021)

Milkstone cleaner is entirely phosphoric acid and available at farm supplies type places. 5L cost around a tenner (some years back - prices seem to be around the same these days), I diluted it down to 10-15% and used a handheld sprayer (a small one) to spray it on to remove rust on a previous car restoration.

Painted it back up with POR15 type paints before top coating - in the engine bay.

HIH

Dibs


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