TheSamba.com :: View topic - Phosphoric acid de-rusting advice/paint bleaching

Tue Jun 14, 2016 6:23 am

A couple friends recommended that before I use POR-15 on the numerous rusted areas on my recently painted 1959 ragtop bug (long story short - began rusting 1-1/2 years after well-known shop did a pan off paint job), that I use phosphoric acid/water/acetone rinse on rusted areas, after wire-brushing the heavy rust off.

I bought a jug of Klean Strip Prep & Etch at Home Depot (see product link below) and put some directly into a spray bottle, without diluting. My friends had recommended that the phosphoric acid be kept wet for ~30 minutes, thoroughly rinsing with water, let dry, then do a light acetone "rinse" to remove remaining water.

The phosphoric acid works much better/faster than Naval Jelly.

HOWEVER, make sure that if the acid gets on any paint, to immediately remove it via water rinse/water-soaked sponge/rag (soap?). In my inexperience, I did not, and noticed weeks later that I have numerous subtle bleach marks on my exterior paint from the acid over-spray, which I'll have to re-paint down the road.

Product link:
http://www.homedepot.com/p/Klean-Strip-1-gal-Phosp...p+and+etch

Best,

Thom

Tue Jun 14, 2016 7:42 am

Actually....your friends are incorrect.

The kleen, strip product is 35-45% phosphoric acid. I did not research the CAS # of the phosphoric acid it uses so I dont know the solution,strength of the phosphoric acid it uses.

But.....at that concentration its similar to Jasco metal etch.

Both products are about 2.5X stronger than average phosphoric acid converters like ospho.

At full cencentration.....they are great at litrally dissoling rust down to the metal. When that is what you want to do.....get rid of crusty rust.......use it full strength.

However......if what you are trying to do in the case of thin, non-crusty surface rust....is to convert the rust to iron phosphate, prevent further rusting...and make an excellent primer base for paints. ...you MUST:

1. Dilute the product with about 40% water....preferably distilled.

2. Wet the rusty area with this solution. ...and let it completely DRY for 24 hours until all rust is converted to a matt black layer.

Not letting the product completely dry....does not allow complete conversion of rust....because the change of state from liquid back to solid is part of the chemical process.

This information is in the tech data sheets of the better phosphoric acid products. Letting it dry less than completely...which generally takes about,24 hours.....results in a weak layer and it will rust again. Ray

Tue Jun 14, 2016 5:18 pm

Although I was essentially just warning about the potential bleaching of paint, I appreciate the additional info even though I've already put the POR-15 on and am well beyond the point of no return.

However, I did space the water/acetone rinse stages ~24 hours apart for complete drying of each stage, prior to applying both POR-15 layers a full day apart even though I may not have been clear...

Best,

Thom

Tue Jun 14, 2016 6:59 pm

catahoula lou wrote: Although I was essentially just warning about the potential bleaching of paint, I appreciate the additional info even though I've already put the POR-15 on and am well beyond the point of no return.

However, I did space the water/acetone rinse stages ~24 hours apart for complete drying of each stage, prior to applying both POR-15 layers a full day apart even though I may not have been clear...

Best,

Thom

In the future....there is no usefulness in a water rinse. Adding moisture into the surface promotes rustimg through the converted surface as iron phosphate is rather porous. Thats why it makes such a good primer. Ray

Thu Jun 30, 2016 10:47 pm

raygreenwood wrote: catahoula lou wrote: Although I was essentially just warning about the potential bleaching of paint, I appreciate the additional info even though I've already put the POR-15 on and am well beyond the point of no return.

However, I did space the water/acetone rinse stages ~24 hours apart for complete drying of each stage, prior to applying both POR-15 layers a full day apart even though I may not have been clear...

Best,

Thom

In the future....there is no usefulness in a water rinse. Adding moisture into the surface promotes rustimg through the converted surface as iron phosphate is rather porous. Thats why it makes such a good primer. Ray

Aloha Ray, so just the acid, let it turn completely black indicating that the rust is gone. No need to use anything else? I have a similar issue on the inside of my door that I want to take care of but was told that I need to rinse the area to deactivate the acid.

Fri Jul 01, 2016 4:39 am

VERTED71 wrote: raygreenwood wrote: catahoula lou wrote: Although I was essentially just warning about the potential bleaching of paint, I appreciate the additional info even though I've already put the POR-15 on and am well beyond the point of no return.

However, I did space the water/acetone rinse stages ~24 hours apart for complete drying of each stage, prior to applying both POR-15 layers a full day apart even though I may not have been clear...

Best,

Thom

In the future....there is no usefulness in a water rinse. Adding moisture into the surface promotes rustimg through the converted surface as iron phosphate is rather porous. Thats why it makes such a good primer. Ray

Aloha Ray, so just the acid, let it turn completely black indicating that the rust is gone. No need to use anything else? I have a similar issue on the inside of my door that I want to take care of but was told that I need to rinse the area to deactivate the acid.

Essentially the phosphoric acid dries, so no need to rinse. Sometimes if it puddled, it will leave a whitish film that you want to brush off. I used Ospho, which is basically phosphoric acid and some additives, then painted Master Series Silver primer right over that, then the rest of the top coats. This link explains about Ospho, and really any Phosphoric acid product, and echos what Ray said.
http://www.ospho.com/directions.htm

Fri Jul 01, 2016 6:13 am

beetlenut wrote: VERTED71 wrote: raygreenwood wrote: catahoula lou wrote: Although I was essentially just warning about the potential bleaching of paint, I appreciate the additional info even though I've already put the POR-15 on and am well beyond the point of no return.

However, I did space the water/acetone rinse stages ~24 hours apart for complete drying of each stage, prior to applying both POR-15 layers a full day apart even though I may not have been clear...

Best,

Thom

In the future....there is no usefulness in a water rinse. Adding moisture into the surface promotes rustimg through the converted surface as iron phosphate is rather porous. Thats why it makes such a good primer. Ray

Aloha Ray, so just the acid, let it turn completely black indicating that the rust is gone. No need to use anything else? I have a similar issue on the inside of my door that I want to take care of but was told that I need to rinse the area to deactivate the acid.

Essentially the phosphoric acid dries, so no need to rinse. Sometimes if it puddled, it will leave a whitish film that you want to brush off. I used Ospho, which is basically phosphoric acid and some additives, then painted Master Series Silver primer right over that, then the rest of the top coats. This link explains about Ospho, and really any Phosphoric acid product, and echos what Ray said.
http://www.ospho.com/directions.htm

Yes..part of it is drying....but the other part of it is polymerization.....all of these have a surface protetctant in them.....and a huge part of it is neutralization.

There is no need....unlike with high solution strength acids.....to rinse phosphoric acid with water. Never rinse it with water.
Rinse with alcohol at weakest or acetone at strongest. Let this is to rem9ve excess amoujts of surface residue. Ray