Paint type sequence

Russ/40

Russ/40

Well-known member
Does it matter where the epoxy primer/sealer is used relative to polyester primer/surfacer? I have done both starting with the epoxy sealer and finishing with the polyester primer, and starting with the polyester primer and finishing with an epoxy sealer coat. Does one provide better sealing of the metal than the other? Probably should have asked this a long time ago..
 
The "politically correct" answer is do what your paint manufacturer suggests.

With that out of the way, most agree that Epoxy grips onto bare metal and old paint substrates better than urethane or polyester paints/primers. The downside is the epoxy doesn't sand well as it tends to ball up when sanded. If the sheetmetal has been toothed up (using a 36 grit Roloc to rough-up the surface or media-blasting the metal to pit-it-up), then the urethane primer/surfacers have more than enough to bite into and there is no need for the epoxy IMO. Any good quality 2K primer/sealer will work almost equally well at sealing the metal from moisture that will cause rust, --unlike lacquer-type 1K primers. Polyesters can stand more talc which assists in high build qualities without 'bridging', and urethanes are are probably easier to sand than all of them.
 
Thanks Brent, Great articulate answer! Appreciate the response.

As a followup, does a Urethane finish adhere better to the epoxy or the Polyester, or is there a difference?
 
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From a hobbiest point of view there needs to be consideration for how you are working on your panels. If you are going to move through the paint work in the near future then filler direct to metal is ok. If you live in humid areas or places with condensing temp changes then you need to think about metal sealing first.

Fillers can absorb moisture and the heat of cure can bring moisture to the metal surface if the conditions are right.

Urethane paint is a very good top coat. It chemically tries to make a very strong, stable single molecule covering the entire surface. It seals the surface and it does not change size in time. It bonds very good to a propperly prepared surface.

Keep in mind that the modern 2K paints do not relie on chemical bonds or melting the previous suface. They bond to propper scratch or holes. For example, epoxy is a totally different chemistry from urethane. When epoxy is curing the suface has holes that the next coat can put it's strands into. Once the epoxy cures the holes close up around the strands locking them in place. This also hoe polyesther filler can adhere to epoxy due the recoat window. The above is based on a conversation with the owner of SPI paints.
 
Kevin in NJ said:
The above is based on a conversation with the owner of SPI paints.
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I am a big SPI user, and think it's absolutely the best. On my build, I am using Epoxy only, no 2K-why? 2K can shrink while Epoxy doesn't. I would not worry about Epoxy either as a Primer, substitute for 2K or Sealer-it does all of the above-
 
35window said:
I am a big SPI user, and think it's absolutely the best. On my build, I am using Epoxy only, no 2K-why? 2K can shrink while Epoxy doesn't. I would not worry about Epoxy either as a Primer, substitute for 2K or Sealer-it does all of the above-
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Why do you say "2K can shrink" yet you feel that Epoxy doesn't shrink? Do you know what causes shrinkage?

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BRENT in 10-uh-C said:
Why do you say "2K can shrink" yet you feel that Epoxy doesn't shrink? Do you know what causes shrinkage?

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Brent,

This fact was bestowed upon me by the owner of SPI, so no, I do not know the Chemical reasons for it. I have seen where it can show up in the appearance of scratches under the the Paint where there were none before. My feeble mind would tend to believe that the cheaper brands of 2K shrink more than others, but application and dry times can also be a factor.

I work for a company that has GM Dealerships, and I go over to the Body Shop as often as I can to learn-they were restoring a '69 SS396 Chevelle for a long time Customer, and the Body Shop Manager and I were talking-he said they were only using Epoxy on that Car, and I asked him why-his response was that it "dries as hard as a Hockey Puck" and was the most stable substrate that he knows of (and they use only Glasurit in that Shop). That, and the recommendations and usage of the Owner of SPI , Barry Kives, are good enough for me-there are many corvette restorers, such as Crazy Horse Rod Shop that only use SPI Epoxy and no 2K, and I wanted to use the best products and techniques possible-

I also apply Epoxy before doing any Bondo work (if I have to use Bondo)-if I Epoxy first, then Bondo, then Epoxy, the Metal can't rust (if I have, say, a pinhole in the Weld)-otherwise, the SPI Epoxy doesn't ball up when you sand it-I actually use it as a guide coat, and it works well-

By the way, epoxy under filler is required for warranty work for Mercedes, Toyota, Land Rover, BMW and Porsches, and that, too, made me think-
 
Well, I want to start by saying I don't have all the answers nor do I know all the solutions however I think you are hearing propaganda from a biased paint manufacturer.

Generally what causes "shrinkage" is due to solvents (thinner, reducer, etc.) evaporating out of the paint as it dries. It really does not matter if the product is a 1K or a 2K, --nor does it matter if it is an epoxy, a urethane, or a polyester, ...if it has solvents mixed in to aid in spraying, it can shrink if not correctly applied!

Now the next scenario that can occur (which I think may be what they are calling shrinking) is called Bridging. This is generally the result of using a heavily talc-ed up primer/surfacer during leveling of the surface. These products generally go by the name Hi-Build Primers. In this scenario, the owner is correct that Epoxy primers won't "bridge" due to the Epoxy's "thin & runny" nature.

With regard to Epoxy required under filler on those vehicles, you are kinda mixing apples with oranges. The new metal in most of those vehicles is a tempered sheetmetal that is different than what you are going to find on a Model A or a 70's GM product. If you go back and look at my post above, you will see where I mentioned that Epoxy sticks to bare metal better than anything if the metal is smooth. That is the "key" why many manufacturers might suggest the use of an Epoxy under a filler simply because the metal is thinner and harder. Grinding on the surface of modern metal to promote adhesion not only takes additional time but creates heat. This step is not necessary on metal that has been "toothed" up either by media blasting, or using a grinding disc to scratch the metal.

Now one other point about Epoxy I would like you to consider is most would agree that Epoxy does not sand very well as its nature is to ball up when sanded. It also does not have very good filling qualities which means another type of filler (Bondo, Putty, etc.) must be used if you are looking for ultra nice leveling (Blocking). The one final thing that your BS Mgr. pointed out about Epoxy is that he is correct that it is harder than a Hockey Puck however when that stuff is thick, it is also brittle. This is NOT something to brag about. Urethanes tend to be more pliable. I have shown people here at my shop where we can powderpaint a piece of aluminum foil and bend it without breaking the paint because that urethane is just like a flexible plastic. Even modern fillers (that are top quality brands) are very flexible and not brittle like old fillers that used to crack and pop out. I promise you that I can take a piece of aluminum foil and dip it into Epoxy one time and after it has dried it will be just as pliable. If I dip the aluminum foil into Epoxy 3 or 4 times while allowing it to dry between coats, it will crack when bent. It's just the nature of it. There are places in the industry for Epoxy however it is NOT a "one size cures all" product. Even the new Epoxy 'Hybrids' that are touted as start-to-finish products have to give up one thing to gain something else. My suggestion is to use only top-quality products that have had plenty of R&D done to them, and then use each product as recommended for each application.

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BRENT in 10-uh-C said:
Well, I want to start by saying I don't have all the answers nor do I know all the solutions however I think you are hearing propaganda from a biased paint manufacturer.
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Isn't it great that there are so many different ways to Paint a Car that will ultimately satisfy the end-user? And, there are many ways to disagree respectfully on the Internet. I will absolutely believe in SPI and Barry Kives, they haven't steered me wrong yet-you might consider trying some of their products, they are priced at a great price point, and I am not the only Customer to be swayed over to the dark side-;)

As for the sanding of Epoxy, you really need to try the SPI, when properly cured, it does not "ball up"-anyway, back to your regularly scheduled, fun, old car discussion!
 
This post has taken on a new life from where I started, but I appreciate the discussions, as I always learn new things.

Now if I might ask another related question. If I complete a panel, fender or whatever, and I leave it in either its final polyester primer, or epoxy, ready for my top finish, does it matter how long the panel sits before I put my final urethane finish on? Can it sit too long? Does the bonding between finishes get better with time, or is there an advantage to limiting the time before the final finish? Is it better to wait to assure the release of solvents?
 
Russ, it doesn't matter how long it sets. The only thing that matters is the surface prep before the next coat of "whatever" is applied.

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