Sandblasting problem

[wrndln]

First, I have sandblasted a lot of parts including fenders, splash aprons and whole bodies, and I mean a lot of items. This is the first time I have seen the problem I am now having. I am attaching some pictures of a pickup tailgate I sandblasted two times in the last week. Both times, the tailgate looked good like all the other items I have sandblasted previously. However, after both sandblasting jobs, a black substance appeared a day or two after the blasting. It must be some form of rust. The black substance looks like it is coming out of some rust pits. I blasted the black area well. This happened to a couple other smaller parts. I have sandblasted a lot of rust pitted parts before and this has not happened before. I stored the parts in a dry area after blasting that had little humidity, so I don't think that is the problem. Note that most of the tailgate is black substance free. Can anyone tell me what is happening and what I can do to eliminate the reoccurring black substance.
Rusty Nelson

[Russ/40]

Try using a finer grit blast material. Looks to be old and deep pitted rust.

[jbond]

Its rust, down in FLA it will happen overnight from moisture in the air. Immediately after blasting, spray or wipe with Ospho. This turns the remaining iron oxide to iron phosphate and delays new rusting. Wipe/dry the excess Ospho off after 10 to 20 minutes. Prime with epoxy (PPG's DP product is good) asap.

[SSsssteamer]

Originally Posted by jbond

Its rust, down in FLA it will happen overnight from moisture in the air. Immediately after blasting, spray or wipe with Ospho. This turns the remaining iron oxide to iron phosphate and delays new rusting. Wipe/dry the excess Ospho off after 10 to 20 minutes. Prime with epoxy (PPG's DP product is good) asap.

Ditto.

[eagle]

First off, when there is deep pitting, I use glass beads that have been in the blasting cabinet for years. They are so broken up and fine, it is almost dust. It will get down into the bottom of the pits and clean them out well. Wipe down with fin-l-wash and prime/seal. Sand lightly if areas need some filling/body work. I'm sure there are lots of ideas but I have had the exact issue you are seeing and it has solved it for me.

[Dennis Pereira]

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Rusty, I keep finding black paint down in the pits under the surface rust. Turns to goo in the tank. I always use a phosphoric solution after blasting.

[Mike Peters]

By any chance was this tail gate lying flat for some time with a bag of sidewalk salt stored on it? I've sandblasted farm tractor rims that have had calcium chloride leaking into the rim for years. They will re rust almost immediately because the CaCl has dissolved into the steel. I don't know the solution to this problem.

[H. L. Chauvin]

Many wonderful responses above.

Today's modern paint has become so sophisticated that if one attends a technical seminar conducted by a top quality industrial or commercial paint manufacturer today, one hears "paint" questions from the audience, and only "coatings" responses from the highly trained instructor.

The old cheap "Paint" term like Mary Carter "Paint", (i.e., buy one gallon & get one free) has been thrown under the bus.

POR, i.e., (Paint On Rust), a chemical sealer, is another subject; however, today's industrial protective coatings on steel have to stop small, minute molecules of moisture from contacting and rusting the steel.

When lead was used in paint, it sealed & worked well to not only seal both wood & metal, but it also killed mildew on wood or steel; plus, when lead paint was used as sizing under wall covering/wall paper, the lead poisoned and stopped live mildew and mold from forming on wall covering/wallpaper.

In my opinion your sandblasting is "unsealing" old metal by removing an old time, old effective "paint" sealer.

Treating this metal, as offered above, immediately after sand blasting should help.

Also, as a side note, sand is not used in finish coats of today's exterior modern latex plaster such as "Drivit", "Sto's" etc. ...... only marble dust.

Many sand deposits contain slight amounts of iron ore which can rust after getting wet, and can cause visible unsightly rust spots on exterior white plaster.

[H. L. Chauvin]

Many wonderful responses above.

Today's modern paint has become so sophisticated that if one attends a technical seminar conducted by a top quality industrial or commercial paint manufacturer today, one hears "paint" questions from the audience, and only "coatings" responses from the highly trained instructor.

The old cheap "Paint" term like Mary Carter "Paint", (i.e., buy one gallon & get one free) has been thrown under the bus.

POR, i.e., (Paint On Rust), a chemical sealer, is another subject; however, today's industrial protective coatings on steel have to stop small, minute molecules of moisture from contacting and rusting the steel.

When lead was used in paint, it sealed & worked well to not only seal both wood & metal, but it also killed mildew on wood or steel; plus, when lead paint was used as sizing under wall covering/wall paper, the lead poisoned and stopped live mildew and mold from forming on wall covering/wallpaper.

In my opinion your sandblasting is "unsealing" old metal by removing an old time, old effective "paint" sealer.

Treating this metal, as offered above, immediately after sand blasting should help.

Also, as a side note, sand is not used in finish coats of today's exterior modern latex plaster such as "Drivit", "Sto's" etc. ...... only marble dust.

Many sand deposits contain slight amounts of iron ore which can rust after getting wet, and can cause visible unsightly rust spots on exterior white plaster.

[modelAsteve]

I owned a sand blasting shop for a few years (That makes me an expert-maybe)
What everyone said is correct. Here are a few new thoughts- wet/ damp sand can cause your problem. It can be damp yet still blast ok. Do not use glass bead- especially old glass bead. The beads can drive rust into the metal. The old bead can have rust in it and you are driving rust into the metal- This is really bad on aluminum. You can clean a part up with the used media then hit it with new/ fresh media.

[Big hammer]

A good time to try ....... Evaporust...... and a good spot the ridges would hold it!

[Dollar Bill]

Plus 1 for the Evaporust solution.

Rust911has a product works like Evaporust but is about half the cost.

[Kevin in NJ]

BAsed on my experiments you are not spending enough time to get the rust out of the pits. If there are any dark spots at the bottom of a pit then rust will come back.
I noticed this with some parts I had done. I had been using a product that was supposed to protect the metal, but I noticed that I got rust flowers from the pit. The existing rust has a supply of moisture and allows for the electrolytic action that increases the rust rate.

The cure, sandblast it longer until you can not see a dark spot at the bottom of the pit. NO the various acid things do not seem to cure it. The rust spot is too thick for the acid to convert all the material.

What I have not done is put the part in an electrolytic rust removal tank and see if that fully removes the rust.

Keep in mind that putting a rust coating or epoxy does not stop the rust from happening. It only hides it. I found this out when taking paint off of parts done a long time ago and finding the metal having a surface rust under the paint.

[Tom Wesenberg]

I agree with what Kevin just said. My Cub Cadet wheel had deep rust pits in the rear wheels from some fool using calcium chloride for weight. I put it in the electrolytic tank for 2 days, then rinsed with hot water, dried it and then gave it a light sandblast, and it came out perfectly clean. I just don't recall if it had a few black spots before I gave it the light sandblast.

[Viv W]

Hi,
I've had this problem before from my sand blasting guys. If it shows light brown or black, as in your photo, then the blasting has not fully removed the rust.
Sandblast it more until it is a clean grey/silver color.
You can also put phosphoric acid onto these areas, but you must keep the place you are treating wet with acid for several hours.Keep putting the acid on and working these areas until there is no trace of black or brown spots. Once you are sure there are no brown or black spots left, give the area a quick wire brushing. If there is any trace of brown rust still coming up with a wire brushing, then continue with the acid until you have achieved clean metal.
One thing you MUST NOT DO, is to attack this area with a grinder and a sanding disc. It was explained to me by my plating shop, that when you try and grind out rust pits, you are actually melting the metal. The high spots are basically melted and pushed over the low spots, thereby sealing the rust, inside the low spots. In time the rust will re-occur.
Viv.

[BRENT in 10-uh-C]

Originally Posted by Dollar Bill

Plus 1 for the Evaporust solution.

Rust911has a product works like Evaporust but is about half the cost.

FWIW, I purchased the Rust911 large concentrate to try, and have been mixing it in 2 gallon quantities in a tub filled with cast-iron, and while it is doing well, it does not seem to last as long as the Evaporust product does/did for me. Since I presently do not have any of the Evaporust product to do a side-by-side comparison, I am only basing this on this observation. Have you actually compared the two products?

[pooch]

More oxygen in the pores of the metal in the affected area than the rest..

[700rpm]

Well Rusty, you have the right nickname. To your problem, the way one of the dangers of sand blasting was explained to me about 40 years ago went like this: when blasting there are tiny peaks and valleys created by the more rotten areas being blasted away before the firmer metal. But that can leave minute rust on the sides of the peaks, and as you continue blasting the peaks get pounded over before all the minute rust is removed. The result is that the peaks flatten out and trap rust underneath. This is similar to what Viv is describing above. And this is one reason why I have never sandblasted sheet metal. It might work fine 98% of the time, but that 2% could drive a preacher to drink.

[duke36]

Check the sand you're using. Some sand comes from ocean beaches (salt laden) and is a problem in concrete mixes, plaster, blasting, etc.

[Dollar Bill]

Originally Posted by BRENT in 10-uh-C

FWIW, I purchased the Rust911 large concentrate to try, and have been mixing it in 2 gallon quantities in a tub filled with cast-iron, and while it is doing well, it does not seem to last as long as the Evaporust product does/did for me. Since I presently do not have any of the Evaporust product to do a side-by-side comparison, I am only basing this on this observation. Have you actually compared the two products?

Yes, I first purchased Evaporust to clean the cooling jacket in my Roadster. It cleaned the block but wasn't much good for anything else afterward and I did not dilute the product.

I purchased the Rust911 product almost 2 years ago and it's still going strong.

So my experience is the EvapoRust costs more and doesn't last as long as Rust911. YMMV