Lost in NJ
11-30-2005, 11:49 AM
As it was explained to me by Dupont.
You need to give paint something to grip to on metal. Smooth metal does not have many places for paint to stick to.
You must etch the metal to give paint something to stick to. This can be done mechanically by sanding or sandblasting or chemically. An etching primer would be needed on metal that had been stripped chemically, but not sanded or sandblasted, so that the paint could adhere to it.
I believe that etching primer will also bind the surface atoms with something that makes it much harder for water to make rust. I am not 100% sure of that point. This can also be accomplished with products such as PickelX 20 or different forms of phosphoric acid etch solutions that are widely available.
The epoxy primer will bond tightly with etched metals forming a seal against moisture.
There are some epoxy primers with chromate that will help prevent rust, but they are getting hard to find and epoxy applied correctly will be an effective seal, or so I understand from reading on this board. Chromate primers are harder to find today because of the EPA regs or so I am told.
So one paint is not better than the other, they each have their uses. It all comes down to reading the tech sheets and hoping to get some correct information on the best ways to use each product.
11-30-2005, 09:01 PM
I went through the debate some time back, and the simple truth is, that either, when used appropriately, will basically yield the same results that we are after (a lifetime of protection).
Just like anything else, there are variables (both in quality and cost) to both etch and epoxy, as well as their use limitations, and user friendliness.
I use, and have always been satisfied, w/ DuPont's Variprime...but now approaching $200 for a mixed up gallon, I'd hardly say that's cheap...on the flip side, there is the Kirker epoxy, that's less than half that for the same amount...so on price alone, you can bet there is a difference throughout, in terms of quality...bottom line, is depending on what you need to accomplish, you can often gamble and come out fine, or certainly spend a bit more on a tried and true product.
Now, my projects (that I complete and care about), are stored covered and indoors...if that wasn't an option, I certainly just go w/ epoxy (and not urethane prime until I was at the stage of final blocking).
Over time, technologies and formulations "change"...not necessarily improve (at least not in all respects)...and you will find plenty of people on both sides of the dilemma, the vehemently support their position to use one or the other, and w/ good sound reasoning, as they both yield the results they're after.
Epoxy seems more universal in it's application over a great variety of substrates...but the same can be said (at least for Variprime) for etches as well (bare metal, filler, plastic)...granted, filler and plastic don't rust (so choosing etch vs epoxy in that application is moot anyway).
Both have been around for decades...in the beginning, epoxy had a hard time sticking, so to help ensure that, you did something to the surface to help w/ that...like a vinyl wash, to "etch" the surface...Picklex, DuPont 5717/5818 etc., all help in that regard.
No doubt, that added step helps in two ways, not only to provide a mechanical tooth, but to also leave an anti-corrosive layer of protection via the zinc/phosphoric acid combo.
Now some may say, that that's just another added, unecessary step...and depending on what your accustomed to, it may or may not be.
Etch primer, in and of iteself, provides that chemical bond, while leaving the anticorrosive chemicals in place...in either case, etch or epoxy, if you intend to block it out, thry'll get urethane primer (in most cases) anyway.
Sure, there are variants of sandable epoxy, or even through in chromated polyester, and kill "3"! birds w/ one stone (etching, sandability, moisture resistance).
Good quality etch, is used for the benefit purpose of putting those chemicals on the metal, that inhibit rust...and is a chemical biting in and adhering (not gluing) to the metal.
Good quality epoxy, is used for the benefit purpose of sealing out moistiure/air from the metal...by an impermeable glue like film.
It seems fair enough to say, that in modern day formulations, both etch and epoxy stick as well as they need to, to a properly prepared surface, and when the paint job is taken care of properly, should outlast us...then again, remember, in the best case scenario, we could refinish our cars utilizing the same procedures that even the best car-maker uses...but as we all know, even in that case, depending where you live and when you drive, even stuff that has an E-coat, is dipped, sprayed w/ whatever etc...still has areas prone to rust.
I've got an abandoned project, 10 years outdoors in NJ, under a leaky tarp. Stripped clean, then shot w/ DuPont Variprime etch, and let to see it's cycle of rain, snow, heat, and whatever else...now, admittedly, there are a few areas of rust cropping up (mostly I suspect from abrasion), but what always surpises me, is that the areas that were heavily pitted, and only wire brushed clean (like the horizontal areas underneath the window trim for the 1/4 windows and door ledges) STILL don't have rust in the pits, after a decade!
So, for me, that's proof enough, that etch, though it's not even a close equal to the performance in water resistance of an epoxy, still does "resist corrosion"...again, there are "good" and "bad" varieties of everything out there.
Ideally, is you can locate a true chromated epoxy, that seems a pretty sound bet as a product to use, w/ the least amount of what "ifs" and additional steps to take...most tech reps will attest to the same.
Having spent what seems to amount to way to long on a project, I ended up using etch (even though I'm still not through w/ the priming stages on the shell)...mainly because that's what I'm used to, and that's the systems approach that I was accustomed to using DuPont products (though they made an epoxy, it was never pitched in the same manner as PPG does, though they have some improved DTM stuff nowadays).
I'm sure...if you look long and hard enough, you'll find people that build $250K plus trailer queens, that use either etch or epoxy as the first layer of primer, and they'll never change that approach...reason being, they have too much to risk on attempting a change to the unfamiliar, and more improtantly, they know it works, whatever it is they're using.
A few years back, I spent a day (had time on my hands) posing the epoxy vs. etch debate to as many restoration facilities I could find on line...and there was nothing definitive one way or the other, but at least justifications for why they did it one way, and an equal split over that.
There truly are endless variables to choosing why to do it one way or the other, and I'm sure that once you come up w/ a rational plan, it's easy to produce a counterpoint argument that the other way (whatever it is) supercedes the previous, and makes more sense.
The only thing for sure, is stay away from lacquer and 1K products...then again, there are the rare times that call for this, and IF someone knows what you're doing, maybe it can be pulled off...epoxy and etch are far more stable, shrinkage is a non issue given the reasonable flash times, and they both add something that undoubtedly protects the metal (whether it be imperviousness to moisture, or chemicals etched as sacroficial).
Also, remember, that a painted car, has alot more than just primer on it (agreed, it's a pretty important first step)...but, considering you'll likely have urethane primer (which is probably more impervious to moisture than etch, but less so than epoxy...in either case, leave unsanded to keep moisture out), and that the final coats following will be hardened base or a urethane clear, you do have additional protective layers coming.
Think about it, if you have penetration beneath your clear and base, down to your primer, you have BIG problems at that point whatever you use...the clear should ideally seal up the paint sandwhich.
Interesting thought as well...we think about the protection of a cured paint job, and use an epoxy or etch as a first layer primer...but, in the end we use a urethane clear as that first line of defense for the system.
Now, urethane clear contains a lot more hardener in it than a conventional build urethane primer...but what's so much the difference anyway...
If epoxy was the end all and be all, wouldn't there be an epoxy clear formuated for commercial automotive refinishing use that supercede all preceding layers?
Primer IS important...but SO ARE topocoats...if your topcoat isn't penetrated, than how much of an issue is it what sort of primer was used underneath, so far as it was compatibile, and remains stable...once the sandwich is breached, it calls for through techniques for a proper repair anyway, and when it comes time to scuff and blend the base, whats a bit more sanding to clean up the metal and start bare at the area in question anyway...I say all this, knowing that my daily driver, supposedly painted w/ the best technology of the factory (ok, I admit it, I'm a skepticist, that there can likely always be a "better" way to do this, but the car manufacturers are too cheap, so the criteria is "best, cheap, reasonbly acceptable way") has plenty of places, where in 5 years from new, had rotted out completely!
So, some areas, by design, rot...and short of the stainless repair option, may well do so again when used in the same circumstances.
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