How clean is clean

[chevman]

The Holdtight question thread has got me thinking, so I did a little research on it, and their claims are impressive.
So how clean is clean enough?

We use SPI epoxy because it sticks better, but most still say its ok to spray epoxy over light flash rust---even barry. OK, Barry's epoxy will take care of some flash rust as far as future problems, but how well does it stick to the metal that has a thin layer of flash rust? Also if you sand the flash rust off, it is still in the original rust pits that were blasted clean, it really takes compressed air to dry those pits. So how clean is clean enough?

Some on the forum say that priming after blasting is ok, as long as the surface was clean and fresh blasting media was used. But as pointed out in the Holdtight literature, there are still tiny bits of shattered media and other tiny bits of contaminants all over the surface, such as rust and paint. So how clean is clean enough?

They say it works better with a pressure washer, and that makes sense if it is going to clean embedded contaminants.

As said before, we use SPI epoxy because it sticks better, which means less paint chipping. But would it stick even better without the problems mentioned above, and just an overall cleaner surface?
https://holdtight.com/products/holdtight-102/

https://holdtight.com/industries/automotive/

https://holdtight.com/wp-content/uploads/2021/02/HoldTight-SDS-8.5.20-FINAL.pdf

 

[Barry]

Flash rust is rust that usually pops up after stripping, and the good thing about flash rust is that it tells you the panel is semi-clean.
The second is flash rust, unlike pit or scale rust, which is not thick enough to hold air and water.
The test I did years ago blasted a new fender and, for about a week, shot water on it every day, morning and night.
It was dark golden yellow, did not clean or sand, shot two coats of epoxy, and the next day cleared because it was going to be on the roof of the building, and all I cared about was the flash rust.
Years later sold the building. And moved, I carefully sanded, and the metal was perfectly clean. Flash rust is gone.

Just to clarify as would never promote spray over rust. Da or red scuff pad will knock off pretty easily; however, metal can never be clean enough, so I will let pros take the clean from here.

 

[Chris_Hamilton]

But as pointed out in the Holdtight literature, there are still tiny bits of shattered media and other tiny bits of contaminants all over the surface, such as rust and paint.

I'm no expert about all of this, though I would think 90-100 psi of air pressure hitting the panel would eliminate most of the media and other contaminants. How much different is that versus pressure washing? I don't know but I don't think it's much. Shouldn't be any different than sanding and what's left on panel at the micronic level after blowing it off. Abrasive media and contaminants would get enbedded because of sanding as well. I would think if there were failures due to that we would have heard about them.

I look at what they say also in the context of them selling their product.

I think if one had they capability to inspect a blasted panel on the micronic level after blasting, then after blowing off with air, pressure washing etc. it would be interesting.

I think blowing off very thoroughly after blasting is mandatory but beyond that? I don't know. Like I said above if it was an issue it would have trickled down through the grapevine so to speak.

Just my completely uneducated thoughts about the question Chev.

 

[Chris_Hamilton]

If someone was so inclined I would think compressed air and a handheld fine wire brush scrubbing the surface would equal what's achieved with pressure washing, without the drawbacks of getting the panel wet.

 

[Toolin]

Flash rust is rust that usually pops up after stripping, and the good thing about flash rust is that it tells you the panel is semi-clean.
The second is flash rust, unlike pit or scale rust, which is not thick enough to hold air and water.
The test I did years ago blasted a new fender and, for about a week, shot water on it every day, morning and night.
It was dark golden yellow, did not clean or sand, shot two coats of epoxy, and the next day cleared because it was going to be on the roof of the building, and all I cared about was the flash rust.
Years later sold the building. And moved, I carefully sanded, and the metal was perfectly clean. Flash rust is gone.

Just to clarify as would never promote spray over rust. Da or red scuff pad will knock off pretty easily; however, metal can never be clean enough, so I will let pros take the clean from here.

Good Info.

 

[Lizer]

I'm no expert about all of this, though I would think 90-100 psi of air pressure hitting the panel would eliminate most of the media and other contaminants. How much different is that versus pressure washing? I don't know but I don't think it's much. Shouldn't be any different than sanding and what's left on panel at the micronic level after blowing it off. Abrasive media and contaminants would get enbedded because of sanding as well. I would think if there were failures due to that we would have heard about them.

I look at what they say also in the context of them selling their product.

I think if one had they capability to inspect a blasted panel on the micronic level after blasting, then after blowing off with air, pressure washing etc. it would be interesting.

I think blowing off very thoroughly after blasting is mandatory but beyond that? I don't know. Like I said above if it was an issue it would have trickled down through the grapevine so to speak.

Just my completely uneducated thoughts about the question Chev.

My issue with only blowing blasting media off is while the coarse media is gone, there can still be a lot of fine dust left behind that doesn't necessarily get blown off. So I like to at least WIPE a part down (even if it's a dry rag) to get any dust off.

 

[dhutton01]

Wiping down blasted metal usually leaves a mess of snags in my experience. The juice isn’t worth the squeeze imho. Blow it off good and hit it with epoxy. Done it that way for 20 years.

Don

 

[crashtech]

Wiping down blasted metal usually leaves a mess of snags in my experience. The juice isn’t worth the squeeze imho. Blow it off good and hit it with epoxy. Done it that way for 20 years.

Don

That's been my problem, too. Snagged fibers all over the panel that look a real mess.
Even without wiping it's gotten to the point where I can't prime blasted parts just once and give them to a customer, they have to be sanded and re-primed to be presentable enough for the public. There's always stray blast media, fibers, or both the first time around.

 

[aviator8]

Wiping down blasted metal usually leaves a mess of snags in my experience. The juice isn’t worth the squeeze imho. Blow it off good and hit it with epoxy. Done it that way for 20 years.

Don

Thought it was just me and my paper towels. Ive been wondering what people use to not get fibers everywhere. I have been getting this when I do my 700 W&G wipe down. Then I take my gloved hands and run over the entire surface to knock loose any fibers, then blow them away.

 

[crashtech]

I'm told a nylon bristle brush and air blower will help, maybe someday I will try it.

 

[Toolin]

Same here. I use a clean 3" varnish brush that has only been used for that specific purpose in conjunction with air brushing in multiple directions. I do the same on primer and epoxy after sanding before wiping. Gets a lot of dust off.

 

[Lizer]

Wiping down blasted metal usually leaves a mess of snags in my experience. The juice isn’t worth the squeeze imho. Blow it off good and hit it with epoxy. Done it that way for 20 years.

Don

This is my experience too. But glad to hear it can be blown off and hit with epoxy. I actually need to do a truck frame soon and wasn't looking forward to the degreasing part.

 

[chevman]

Thanks Barry for your clarification on priming over flash rust.
Its interesting to hear what others are doing, and of course, thats one thing that makes this site so good. I just wanted to check and see if anyone was doing something special and keeping it to themselves LOL.

I bought a SS wire brush for this purpose once, but it wasn't right size or shape, and the wire was too course. I still think it would give better adhesion, and maybe the nylon brush Crash suggested would do the trick. Adhesion is good without it, but always looking for improvement.
Thanks guys.