Corrosion performance: chromated epoxy vs. chromate free

[jsampson]

I am currently in the process of stripping the asphalt undercoating on my 1991 318is. It appears that the primer underneath is the characteristic greenish-gray of a chromated aircraft style paint. There are several areas that need rust repair including some hidden under the undercoating. Current plan is to go to bare metal on the body and underbody.

Glasurit has a chromated epoxy primer 801-703 (SDS) which is in stock near me. I’m sure it'll be / has been discontinued soon, but I can still get it. I planned on using glasurit products at least for the base/clear.

Do you expect significant improvements in corrosion performance with not only an epoxy but a chromated epoxy compared to the SPI epoxy? There has to be a reason airplane paint holds up so well. Of course the glasurit costs about twice as much so if it’s not any better I’d go with the SPI. Of course I will take all safety precautions in the SDS if I use the chrome.

thanks!

 

[anotheridiot]

This non chrome stuff makes me sick. Our current deal for Lycos has us using the new chromate replacement stuff. We can buy the chromated, (stronium chrome, not hex chrome) for 120 a gallon kit. But the pencil pushers usually do aerospace work and want us to use the N (trivalent chrome) which delivered is 340 per gallon kit. Yeah, that is 3 quarts of paint and one quart of hardener. It took a while to buy into the stronium chromate stuff, even longer for the N. These will be the on the ground cameras locating the satellite in space. They require this N stuff on the outside of the airplane, the Stronium chrome(replacement) can go on the inside where the people are.

do people realize when I destruct my hex chrome obsolete chromate tank, I adjust the ph, add sodium bisulfite, raise the PH slowly back to 7 and the sludge that is the result of that process has been converted to trivalent chrome. This lowest form of chromium is what is the current specification.

So you will probably be able to get chromium based stuff, but it wont be the same stuff and I can tell you the answer here will be only use SPI.

 

[jsampson]

Looks like the SDS says chromic acid, strontium salt.

Not so much looking for a “which is better” type answer, rather wondering if the addition of the chromate in the glasurit product gives corrosion protection that is significantly more than what the SPI primer can offer being chromate free.

 

[Chris_Hamilton]

@jsampson BMW's of that era and most (all) others OEM's were not using aircraft grade epoxy primer. Some form of acid etch, usually catalyzed and baked, is what they used. Sikkens Autocryl 3+1 or the Sikkens OEM equivalent of that is what was used.
I've used a lot of different manufacturers products over the years and I can say that SPI Epoxy is as good as any Epoxy on the market. Stuff you are talking about I've used and adhesion is not as good as SPI. Effective corrosion protection involves 2 main things. Film build and adhesion. You need to have enough millage (film build) so the coating can protect. Secondly and just as important you need to have superior adhesion. SPI has the best adhesion of any epoxy I've ever used. (Youtube SPI Epoxy torture test to see for yourself). Use what you want but in a practical refinish environment SPI is a superior product to any refinish epoxy out there. Plus it's like half the cost of Glasurit.
I worked for 2 'Euro" shops over the years. Learned a lot, got to go to the Mercedes Benz school to train. Where I first learned of epoxy. But one thing that bothered me about working for both those owners was that they were "Euro Snobs". If it wasn't European it wasn't good in their eyes. And that really wasn't true, but they were happy getting something of equivalent quality and paying twice as much. Like I said do what you want.

Oh and not trying to be a smarta** but ask yourself this question, if the OEM product was so good how did rust form when it had the OEM coating and undercoating?

 

[anotheridiot]

@jsampson BMW's of that era and most (all) others OEM's were not using aircraft grade epoxy primer.

It all started with Military paint. The formula is not that difficult to find, the expense of testing of every batch to stay on a qualified products list for the military is a different story. The downside from the real idea of it is no military paint is designed to be direct to metal. There are all required pretreatments PRIOR to the use of the primer, if some of those are not available, you are required to use wash primer. For steel, its iron phosphate. Now, I have a concentrate I helped develop with a chemical plant with the good stuff, (like old motor oil) Molybdate in it. Power wash the bare steel, add the cleaner and phosphate, let it dry and you get the blue toned iron phosphate. Since that is not likely with automotive guys, it was wash primer first. But since I highly doubt anyone would want to spray it before epoxy and deal with seams being trapped with liquid, its a moot point, but that is where Military started a bunch of this, and some people believe if its good enough for the government, its gotta be the best right? Guarantee it beats scratching with a da.

So, sure, some auto paint guy finds the formula and decides to sell it for cars, it was not like a new idea.

BMW was probably one of the first automakers that actually washed their cars thru the tanks that most cars roll thru now adays, so maybe the phosphate was under it already.

 

[jsampson]

Exactly its a play on words, people thinking of the outlawed stuff used in the late '80s and early 90s

Good to learn this - not all "chrome" primers are created equal.

BMW's of that era and most (all) others OEM's were not using aircraft grade epoxy primer.

I'm sorry I worded my question in a way which was unclear. What I meant to convey was that I recognized that greenish-gray appearance from aircraft style primers and suspected there was some kind of chrome pigment in it for that reason. Not that my car had somehow been painted with an aircraft paint. But then I was interested in learning if a chromated primer would add additional corrosion performance over epoxy alone and therefore be a good option for my project.

SPI has the best adhesion of any epoxy I've ever used.

This is exactly the type of answer I was hoping for. In my mind the SPI and the glasurit were *both* epoxy primers to begin with and I had no reason to suspect one was better than the other, but the glasurit had this extra corrosion performance element additive. I wanted to be sure that I felt good about the choice I made if I decided not to get the chromated primer and then have to spend the next 20 years wondering if it would have been better if I had used the glasurit instead.

It seems that because the adhesion of the SPI is superior it should have similar anti-corrosion performance to the glasurit product in addition to it's other advantages that I've read about as far as easy to work with, easy sanding, etc. Not to mention the obvious value.

Oh and not trying to be a smarta** but ask yourself this question, if the OEM product was so good how did rust form when it had the OEM coating and undercoating?

Honestly it worked pretty well for 28 midwestern winters. If it wasn't for some moderate collision damage I wouldn't be repainting the car at all, I would have just polished it. But now I'm down the rabbit hole of searching for "perfection" and rooting out every little bit of corrosion. It was my first car and I'm hoping to drive it for the next 28 years as well. Really the only corroded places are where the finish was broken for one reason or another. In one place I think it bottomed out. In another place there was a dent. In one corroded area there was a clogged sunroof drain that constantly pooled water above it.

 

[jsampson]

SPI has the best adhesion of any epoxy I've ever used. (Youtube SPI Epoxy torture test to see for yourself). Use what you want but in a practical refinish environment SPI is a superior product to any refinish epoxy out there. Plus it's like half the cost of Glasurit.

Just watched the torture test video. Very impressive.

 

[Barry]

To explain myself, I just had time to explain and thought about it and deleted the post after a few mins.
I'm going to in a moment delete my first post as something about this strikes me a weird, and I will leave at that.
All I have to say if a product has lead in it, it will be on the MSDS and the label because, in a roundabout way, that is what they're trying to imply, bs.