Metal surface prep - Settle a debate

[Benny]

There is a pretty sprited discussion going on one of my other message boards about appropriate surface prep for epoxy primer. A guy who once worked as a a Tech Rep for BASF has stated that a blasted metal surface or an 80-grit metal surface it too rough for good epoxy adhesion. His recommendation is to effectively smooth out a blasted surface with a 180 or even 220 grit sanding, before applying epoxy.

He maintains that the epoxy adhesion relies on a combination of chemical and mechanical bonding, and that the primer will lie across the peaks, and create a terrible bond. I understood that epoxy used a purely mechanical bond to create adhesion, and that a blasted or 80-grit surface would be the ideal surface.

Is there any truth to his statements, and if not, what is the technical rebuttal to them?

Thank you

 

[Barry]

Sure there is some truth to his statement for the basf product, there are a 100 ways to make epoxy and neither way is wrong as the company may be making it for a specific purpose.
SPI epoxy, will stick to Aluminum and steel without sanding as long as perfectly clean but we request 80 grit on bare metal to compensate for anything that was missed and perhaps that may also be the difference between a stone hitting the car and there is no chip vs getting a chip. Can't hurt and can only help!
Best thing to do is follow the tech sheet of what product you are using.

 

[crashtech]

Honestly the advice from BASF sounds crazy to me. I've never seen better adhesion than on top of a blasted surface. I guess what that means to me is to avoid the use of BASF products. If it can't flow into an 80 grit scratch or 1 mil profile blasted surface, there is something wrong with it in my estimation.

 

[shine]

well cool . the starblast is somewhere north of 100 grit . but then i wash with soap and water and a paint rep is going to say not to .
bottom line for me is i want the epoxy to " glue " the job together . being able to build and sand is just a bonus . be careful what you read out there . there is some true bullshit going on . i now tell people to ask a painter not a forum .

 

[Benny]

Sure there is some truth to his statement for the basf product, there are a 100 ways to make epoxy and neither way is wrong as the company may be making it for a specific purpose.
SPI epoxy, will stick to Aluminum and steel without sanding as long as perfectly clean but we request 80 grit on bare metal to compensate for anything that was missed and perhaps that may also be the difference between a stone hitting the car and there is no chip vs getting a chip. Can't hurt and can only help!
Best thing to do is follow the tech sheet of what product you are using.

Thanks for the feedback, guys. What about the claim that the adhesion is a combination of mechanical and chemical bond? As I mentioned, I previously believed epoxy used surface tension to create the bond, no?

The problem with the "ask a painter" method is that for an novice (like me), you usually get an answer like "Most people don't do it like this, but it works for me.....painting is a very personal thing"....that just makes your confusion worse. At least with a forum like this one, you can base your next step on multiple responses. If 7 out of 10 guys agree....or at least don't DISAGREE, then you feel pretty good going forward.

 

[jeremyb]

Thing is, always follow the manufacturer's recommendation for whatever product. I wouldn't trust a BASF rep to tell my how to use SPI epoxy.....and vise versa.

 

[shine]

"Most people don't do it like this, but it works for me.....painting is a very personal thing".

wrong answer. not a painter but a wannabe . just as Jeremy said above . you have to follow the tech sheets. period. in the old days you could fudge a lot. try that with today's urethane's and you'll learn how to make jelly .

 

[metalman]

Honestly the advice from BASF sounds crazy to me. I've never seen better adhesion than on top of a blasted surface. I guess what that means to me is to avoid the use of BASF products. If it can't flow into an 80 grit scratch or 1 mil profile blasted surface, there is something wrong with it in my estimation.

I agree completely.
It sounded to me from the OP that the "rep." made a sweeping generalization regarding "all" epoxies. That would be ignorant at best.
The only epoxy product I ever used that specified a particular blast profile is a firearms finish known by the name Cerakote. A good example of following the technical of the manufacturer for best results.

 

[Barry]

Thanks for the feedback, guys. What about the claim that the adhesion is a combination of mechanical and chemical bond.

True to a point but here is the problem with reps, they do not get to call R&D, so they hear everything from the sales manager and most of it are twisted half truths.
I know people in the big companies and it is a rare occasion they ever talk to a rep, if they did they would never get anything done.

 

[crashtech]

One thing I do know, the science behind adhesion is well above most of our pay grades, look up some articles on it and you'll see what I mean.

Also, I don't think there is actually any "chemical" adhesion going on with polymer coatings, since the definition of chemical adhesion means to create a new chemical compound at the interface of the materials. I don't believe there are those sorts of chemical reactions going on with epoxy. But there are other sorts of adhesion beyond mechanical and chemical which may be in play, I'm not well studied enough to know.