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Re: Paint exterior bare metal door

Ordjen: You said:


The average exterior oil paint will get a dusty haze (oxidation) on the surface, making it look unsightly.

So, is this what you'd call "oxidation"?

Even if the paint were another colour, like red, yellow or blue. Would you call the formation of a white dusty powder on the surface of the paint "oxidation"?

Re: Paint exterior bare metal door


Your picture shows old fashioned chalking, a washing away of the poor quality pigments of a cheap paint. I remember that stuff well and how it would wash down onto the red brick below it, creating an unsightly, difficult to remove mess.

What I call oxidation occurs on any color paint exposed to the sun and weather. I remember having painted the south facing metal door to my garden shed with Moore's dark green exterior alkyd gloss oil paint. After a year or so, it became dulled by a whitish haze which could be removed by cleaning with an automotive type cleaner.

The one advantage an acrylic has is that it generally will not do this. However, it is a trade off with more brush marks and that sticky quality acrylics have.

My former house also had 1970's generation aluminum siding which, after a year or two, would change from a dark colonial mustard color to a blah , lighter tan. An extreme power washing would blow away the oxidation and the original color would return - for awhile. I finally painted it with 100% acrylic house paint from Pittsburgh Paint and the problem was solved!

Does the paint color matter as to haze color? Regardless of the initial color, the haze is whitish. This is not just white pigment being washed off a poor quality paint.

Re: Paint exterior bare metal door


Besides chalking, I know of no process that causes paint to deteriorate to form a whitish powdery film. I thought you were talking about chalking.

So, I Googled "paint oxidation" in the hopes of learning what you mean by that, and the ONLY kind of paint oxidation I found on the internet concerned damage to AUTOMOTIVE paints.

I'm sure you understand that oil based paints require oxygen to cure. However, once cured, I'm aware of no process that causes oil based paints to be damaged by the oxygen in the air, causing the paint to deteriorate to a white powder. To my knowledge, only chalking caused by UV light from the Sun does that.

Can you link to a web site that explains, or at least describes, oxidation of house paints?

Is this a term you've heard other people use in connection with house paints?

Re: Paint exterior bare metal door
Nestor wrote:

Ordjen: You said:

So, is this what you'd call "oxidation"?

Even if the paint were another colour, like red, yellow or blue. Would you call the formation of a white dusty powder on the surface of the paint "oxidation"?

I think we say "chalking" when the vehicle breaks down slowly and allows the pigment and solids to slough off. Some paints are called "self-cleaning" meaning that they chalk off readily and the surface is always very fresh-looking. Mainly made very light tints. But you don't want to paint your brickmold on a brick house with that type of paint or the chalkiness runs down and discolors the walls below. We have all seen this occasionally, I'm sure; The brick house with the window frames bleeding white down the walls.

Re: Paint exterior bare metal door

Yes, I understand what chalking is:

The question was whether or not it's appropriate to paint an exterior door with a FLAT alkyd paint.

Ordjen's feeling was that an alkyd paint, especially a flat one, would "oxidize". I believe he meant "chalk".

But, chalking is caused by UV light from the Sun breaking the paint binder down and releasing pigments from it, and so it's highly dependant on the conditions the paint sees in service.
If there's a long overhang over that exterior door that keeps it in the shade, or if the house is shaded by trees in the yard, for example. Or, if the door is on the east or north side of the house that gets less direct exposure to sunlight, or if it's in a northern state then chalking won't be a problem.

So, I think I stand by my original advice, which was to use an exterior flat alkyd paint cuz:
1) alkyd paints dry to harder and more durable films than latex paints, and
2) by opting for a flat paint, then you save yourself the work of having to scuff sand a glossier paint down when it comes time to repaint that door.
3) oil based paints do chalk more than latex paints, but it depends a lot on where you live which will determine if that's even a concern. Where I live, we don't get enough intense sunlight to cause paint to chaulk. I rarely see chalking on paint where I live, and when I do see it, it's on old paint that's go no protection from the Sun at all.

Re: Paint exterior bare metal door

And if all houses had giant umbrellas over them, we might never have to paint them again! :)

OK, we might be spliting hairs over the term oxidation. The last time I took chemistry in college, Nixon was president, the Periodic Table had about a dozen fewer elements, and the Watson-Crick DNA spiral had only recently been discovered! Does that date me?

I am short on chemistry, but long on experience and horse sense. The school of hard knocks yields unforgetable lessons. As stated, I have seen high quality, name brand alkyd exterior enamels haze over when exposed to the elements. At my present house, I elected to paint the front door dark green with a glossy acrylic paint, precisely because it is exposed to full sunshine and occasional rain. Six years after application, it is still dark,shiny green.

I chose gloss paint because it makes dark colors "pop" on trim. It is also much easier to clean when Frau Ordjen gets on my back about the schmutz on it! :)

Re: Paint exterior bare metal door


I agree with you that oil based paints "chalk" more than latex paints. However, you need to understand that this is entirely because the oil based binders in oil based paints aren't transparent to UV light, and in absorbing UV light (and deteriorating as a result) they protect the substrate from UV light.

The binder in exterior latex paints, Plexiglas, on the other hand, is nearly perfectly transparent to UV light from the Sun. As a result, it doesn't deteriorate from exposure to UV light. But, what it does do is allow that UV light to pass right through it and deteriorate the substrate that the latex paint is applied to. If you have a METAL door, then the metal of the door won't deteriorate from exposure to UV light, and so using a latex top coat over a metal door ensures that neither the metal nor the latex paint will deteriorate from exposure to UV light. But, that also means you still have a soft paint over your door that will show wear and tear and mark up more easily.

However, if the substrate is wood, then painting with a latex top coat doesn't really help too much because the latex top coat of paint allows UV light to pass through it easily, and that same UV light then deteriorates the wood the paint is stuck to. Ditto if a latex primer is used under the top coat. The result is that perfectly good latex paint peels off of wood because the wood itself has deteriorated to the point where the latex paint (or primer) won't stick to it anymore. Thus, latex paints don't chalk like oil based paints, but what they do is allow intense UV light to pass through them easily so that the wood substrate deteriorates, and the paint (or latex primer) peels off as a result of the UV deterioration of the wood substrate.

So, under similar circumstances, oil based paints chalk, whereas latex paints crack and peel as the wood substrate they're stuck to deteriorates from exposure to UV light. That's two completely different phenomenon, both arising from the fact that oil based paints aren't transparent to UV light, whereas latex paints are.

You can say: "Well, if this is an exterior METAL door, why not use a latex paint to avoid chalking entirely and thwart the Sun's efforts to deteriorate either the paint or the substrate." I agree that using a latex primer and paint would do that, but you'd still be left with a soft primer and paint on the door that won't stand up to wear and tear well. By using an oil based paint, you get a harder film that will better stand up to wear and tear, but which is also more susceptible to UV light.

I guess the original poster now knows enough about all aspects of this problem to walk away with enough knowledge to form his/her own opinions on how to proceed. If there's a lot of tree (or other) shading to prevent chalking from UV light, then I'd opt for the exterior alkyd paint. However, if the door is unprotected from the Sun, then I'd opt for a higher gloss exterior latex paint and know that the homeowner will have to scuff sand that door before repainting it.

Re: Paint exterior bare metal door


I have never heard the acrylic resin referred to as "Plexiglas". Plexiglas is the tradename owned by Rohm and Hass for its solid acrylic sheeting. It was,by the way, invented in my wife's home town in Germany. (Just an aside)

I do believe Rohm and Hass ( I think it is now just Rohm) is the biggest supplier of acrylic resins to the paint industry. They publish an E-Mail newsletter for the painting industry to which I subscribe.

You seem to feel that acrylic paints are somewhat inferior at proteting the wooden substrate? My experience is that acrylics are vastly superior to oil paints for general exterior housepainting. They allow the house to breath, are far more color fast and its elastic nature allows it to flex with the wooden substrate better.

Being from the old school, I still like oil primers for their ability to not raise the grain, to seal in tannins and other stains and to penetrate deep into the wood's fiber. That being said, the newer acrylic primers seem to be approaching the performance of oil primers.

I have used some of the new Behr Ultra acrylic self-priming housepaint on a pergola I built three years ago. It has just come through its 3rd Oregon winter looking great. It got a "prime" coat of the paint and a finish coat, with minor touch-up due to construction dings during assembly. Apparently, its nano technology is not just some advertising agent's hype.

Re: Paint exterior bare metal door

Yes, good quality latex paints are in fact made of the same kind of plastic that Plexiglas is made of, namely "polymethyl methacrylate".

The Rohm & Haas Company was the largest producer of polymethyl methacrylate resins to both acrylic floor finish manufacturers and acrylic paint manufacturers prior to it's purchase by Dow in 2009. Rohm & Haas is now a wholly owned subsidiary of the Dow Chemical Company.

The Rohm & Haas Company got it's start producing synthetic tannins for the leather tanning industry, but only became well know with their discovery of a way to cast polymethyl methacrylate resins in place to form sheets, which they subsequently sold under the trade name "Plexiglas".

Tiny particles of polymethyl methacrylate, which is the plastic that Plexiglas is made of, are what good quality latex paint binder resins and acrylic floor finish resins are made of. A "micron" is a thousandth of a millimeter, or a millionth of a meter. Your average human hair is about 100 microns in diameter. The smallest thing that can be seen with the naked eye is 20 to 30 microns in diameter. A red blood cell is about 5 microns in diameter. In normal latex paints, the acrylic binder resins will typically be from 1/10th to 1/2 of a micron in diameter. It is the extender pigments in primers, which are typically from 10 to 40 microns in diameter that plug up the porous surface of drywall and bare drywall joint compound to prevent these surfaces from absorbing primer. Consequently, it is the extender pigments that both make the primer a "sealer" (by plugging up porous surfaces) and a "primer" by making the primer dry to a matte gloss so that the subsequent top coat of paint sticks better. In paints that are also primers, the only difference is that the acrylic resins used aren't a fraction of a micron in diameter, but many microns in diameter, and it is these large plastic resins that plug up the surface porosity of drywall and bare joint compound.

Here's how latex paints work:

Good quality latex paint consists of a SLURRY of hard, clear plastic blobs (called "binder resins") and coloured, clear, transluscent or white particles (called "pigments") suspended in a solution of water and a water soluble solvent (called a "coalescing agent"). When you spread that slurry on a wall, the first thing that happens is that the water evaporates. The clear, hard plastic blobs then find themselves surrounded by the coalescing agent at ever increasing concentration. As the concentration of coalescing agent increases, the clear hard plastic blobs dissolve (kinda) in the coalescing solvent to form soft mushy blobs of plastic. The forces of capillary pressure and surface tension which cause small droplets of water to coalesce into large rain drops in clowds then come into play and cause each of those soft mushy blobs of plastic to stick to and pull on each of it's neighbors to form a CONTINUOUS film of soft mushy plastic with the pigments suspended inside it very much like the raisins in raisin bread. The coalescing solvent then evaporates from the latex paint film, filling the room with that "freshly painted" smell, and the plastic film then hardens up to the original hardness of the plastic blobs when they were still in suspension in the slurry.

Primer/Paints are exactly the same, it's just that the blobs of acrylic plastic are very much larger than in normal latex paints.

This is why:

1. Latex paints darken slightly as they dry. Incident light reflects and refracts off of every plastic/liquid interface in the paint film, creating scattered light, just like a clowd, snowbank or the head on a beer. Your eye sees that scattered light as the colour "white". As the plastic blobs coalesce to form a continuous film, there are progressively fewer and fewer plastic/liquid interfaces to reflect and refract incident light, and the result is a reduction of the amount of white light produced from the latex paint film, and an effective darkening of it's colour.

2. The fluid used in the tinting machine used to tint paints is glycerine. They use glycerine because it's equally soluble in both water and mineral spirits, so the same colourants, and hence tintint machine, can be used to tint both latex and oil based paints. Since it's important that the concentration of coalescing agent in latex paint get to be high enough to cause the plastic resins to dissolve (kinda), you can ruin latex paint by adding too much colourant in the tinting machine. That's because glycerine is even slower to evaporate than the coalescing agent (typically "Texanol" made by the Eastman Chemical Company). The result is that the more glycerine there is in the paint, the lower the maximum concentration of coalescing solvent can be. This is why you can ruin latex paint by over-tinting it. Excessive tinting will result in too much glycerine in the paint which results in a lower maximum concentration of coalescing agent, which in turn prevents the plastic resins from softening up as much as they need to to form a continuous film.

(By contrast, oil based paints, (including alkyds and alkyd based polyurethanes) have a much more robust film formation mechanism, and will form a proper film even if applied outdoors in the middle of a Canadian blizzard. The oil based paint will remain tacky until spring, but once the warm weather arrives, it will form a proper film just as through it had been applied on that warm spring day.)

3. Since the last thing to occur in latex paint film formation is the evaporation of the coalescing solvents from the latex paint film, you can tell when film formation is complete in normal latex paints by the "freshly painted" smell in the room. Since that smell is caused by the coalescing solvents evaporating out of the paint, once that smell subsides, film formation is complete.


In a previous post, you said that you didn't like using latex paints on wood because:


I hate acrylic paints on doors and fine woodwork! ... and they retain that gummy feeling which makes them want to stick where surfaces meet.

Actually, in North America, latex paints are made from 3 different kinds of plastics:

1. Polymethyl methacrylate, which you probably know better as "Plexiglas". Paints that are made with Plexiglas binder resins are referred to as "100% Acrylic" in the industry. So, if you see a can of paint with "100% Acrylic" on the label, it means about as much as a package of meat with "100% Cow" on it's label. There are very many "100% Acrylic" resins made from Plexiglas for everything from latex paints to acrylic floor finishes to acrylic grout and masonary sealers to acrylic nail polish for the ladies, and you get what you pay for. So, the wording "100% Acrylic" on a can of paint is more intended to impress than inform. You need to look to the price to tell if you're buying prime rib or dog food.

2. Polyvinyl acetate, which you probably know better as "white wood glue". "PVA" resins are used to make general purpose latex primers and budget priced interior latex paints. In the industry, primer or paint resins made of white wood glue are called "Vinyl Acrylic" resins, or more commonly as "vinyl acrylic copolymers" cuz they'll mix in other kinds of plastic monomers when making the PVA resins to improve the paint or primer's characteristics.

3. Styrenated acrylics. Paints that are made from styrenated acrylic resins have good gloss, but break down from exposure to UV light, so they're used only for interior latex paints. Only about 10 to 15 percent of latex paint production in North America is of paints made with this kind of plastic.

That "gummy feeling which makes them want to stick where surfaces meet" is called "blocking" in the industry, and paints that don't do that are said to have "good blocking resistance". Generally, blocking is only a problem with 2.) Polyvinyl acetate, or white wood glue based primers and paints. That's simply because adhesion turns out to be a hard thing to engineer out of the polyvinyl acetate molecule. So, PVA primers and paints remain slightly sticky even when they're fully dry, just like white wood glue. And, just like white wood glue, PVA based primers and paints also loose their hardness and adhesion due to prolonged exposed to moisture or even very high humidity. So, someone with sweaty hair that rests his head against a PVA painted wall will find that their hair sticks to the wall, both because of blocking, and because the moisture in the hair has softened the paint film. When PVA budget priced interior latex paints are used in bathrooms, the result is that the paint inevitably peels on the bathroom ceilings and high up on the bathroom walls because of the high humidity and shower spray, and that's often misdiagnosed as being caused by insufficient prep work prior to painting. Really, the problem is that the paint used wasn't suitable for wet or highly humid conditions, and that misdiagnosis of the problem undoubtedly causes no end of grief for homeowners that know that they did everything they could have done to properly prep their bathroom before painting it. Paints made of Plexiglas binder resins have very much better blocking resistance and are what's used in paints specifically made for bathrooms, like Zinsser's PermaWhite Bathroom paint.

Yes, I do think that oil based paints are superior to latex paints in many respects. Oil based paints form harder stronger films and have a much more robust film formation mechanism. It is in fact hard to prevent an oil based coating from forming a proper film. By "oil based" I mean drying oil (like linseed oil), varnish, alkyd and urethane modified alkyd (also called "polyurethane").

Re: Paint exterior bare metal door

You can learn more than you need or want to know about latex paints by spending some time at the Paint Quality Institute's web site at:


Specifically, click on the Media Center link, then click on "PQI Publications", and then on "Continuing Education" to get to this web page:


If you download and read the two PDF files entitled:

The Ingredients of Paint and Their Impact on Paint Properties


How Colour is Affected by the Ingredients of Paint

and understand what you've read, then you'll know more about latex paint than 90% of the people working in paint stores. (And, I can probably answer some of your questions.)

PS: You don't need to know the rest...
The Paint Quality Institute was established by, and is funded by, the Rohm & Haas Company who are the largest manufacturers of polymethyl methacrylate plastic in North America. (In the world, it's the Akzo Nobel company, which partially gets it's name from Alfred Nobel, who discovered how to make nitroglycerine much safer to handle, and sold his nitroglycerine under the trade name "Dynomite".) Since polymethyl methacrylate is used in various forms to make Plexiglas, top quality latex paints, acrylic floor finishes, grout and masonary sealers, adhesives, films, foams and even nail polish for the ladies, the Rohm & Haas Company is not a well known name, but supplies the feed stock to many well recognized companies like Benjamin Moore, Diversey Lever, Tile Lab and Clairol (to name only a few) who make the products we use from polymethyl methacrylate resins. The intended purpose of the Paint Quality Institute is to inform and educate painters, architects and consumers (presumably, people who drink paint) about the benefits of using top quality latex paints. So, the Paint Quality Institute has a web page and issues an o n l i n e newsletter each month dealing with paint issues. You can receive that newsletter free of charge by subscribing to it at the Paint Quality Institute's web site at www.paintquality.com

In 2009, the Rohm & Haas Company was purchased by Dow Chemical, but is being run as an independant company, and is therefore free to continue funding of the Paint Quality Institute from it's own profits.


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