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Turkmenbashy 01-17-2009 08:55 PM

Oil, Latex, Acrylic... How to Choose?
I'm repainting a house (fascia, windows, porch ceiling). I some places, the old paint is old, cracking and can be completely removed with a little work. In other places, the surface has multiple layers of old paint and I cannot scrape off the paint down to the bare wood. However, I have scraped until no paint can be chipped off and I have sanded with a 220 grit paper the remaining old paint surface.

Now...what type of paint should I use? Oil, latex, other? What types of uses do you typically use one or the other? Does it matter what type of old paint is still on the surface? How do I know what kind of paint the surfaces were painted with before?

Nestor_Kelebay 01-17-2009 10:50 PM

1. How to tell what kind of existing paint is on the wall:
A) Solubility:
Latex paints are much more soluble in strong solvents like acetone, nail polish remover (amyl acetate), lacquer thinner (which is mostly toluene), xylene, etc. than oil based paints. So, with a little experience, you can tell what kind of paint you're dealing with by how quickly these solvents dissolve it. I've been told that Goo Gone (which is petroleum distillates) dissolves latex paints but not oil based paints, but I've never checked to confirm.
B. Brittleness
Oil based paints tend to dry to harder films that are more brittle. Latex paint films always retain some flexibility and elasticity. So, if the paint is peeling, break a chip off and bend it. If it's brittle, it's oil based paint. If it bends somewhat before breaking, it's a latex paint.

2. Where to use Oil based paints and latex paints:

LATEX paints are best used where high durability isn't important, like on walls and ceilings. These areas might get the odd scrubbing to remove a stubborn mark, but for the most part generally don't have to stand up well to wear and tear.

OIL BASED paints dry to harder films than latex paints and should be used where it's necessary that the paint stand up to a lot of wear and tear. For example, use oil based paints on floors, shelves, table tops that are painted, window sills and mantles where you keep things, etc.

3. Do I need to scrape all the old paint off?
No. There is no reason to scrape off old paint that is sticking well only to replace it with new paint that will stick equally well. Simply sand the edges of the spots of old paint so that those edges don't show through on the new paint. This is called "feather edging" and my building is made of brick, so I'll let one of the professional painters in here talk about how to do it.

4. What kind of paint should I use?
My own feeling is that exterior alkyd paints stand up better outdoors than exterior latex paints. A lot depends on the quality of the paint you buy in each case, but if I were painting over wood outdoors, I'd start with an exterior alkyd primer over the bare wood (don't prime over paint, just sand the old paint's surface when feather edging). Then, I would paint with a FLAT exterior alkyd paint with two coats of an earth tone colour (like mustard yellow, reddish brown, chocolate brown, raw umber, or black, or any colour you can make by combining these colours in any proportion).
The reason for using a FLAT paint is that any subsequent coat of paint in the future will stick well to it without sanding. That saves a lot of work when the time comes to repaint.
The reason for sticking with "inorganic colours" is that all of them (except black) are the synthetic equivalent of the pluverized rocks that artists have been using for millenia to colour their paints. Rocks are good at being opaque, but even better at being old. The rocks in your front yard are hundreds of millions of years old. In order for something to be that old, it HAS TO BE EXTREMELY CHEMICALLY STABLE. (Otherwise it would have decomposed by now and there would be nothing left of it.) That chemical stability translates into excellent resistance to fading from UV light. If you've ever seen a rock on the ground, you'll notice that after cleaning and drying, the bottom of that rock looks no different from the top. That means: it wasn't faded. It might have sat in the sun that way for thousands of years, but the sunny side won't have faded AT ALL, and will still be identical in every respect to the bottom of the rock. Also, rocks are good at being opaque. So much so that transparent rocks are, for the most part, gem stones, and are highly valuable because of their scarcity. If you pulverize an opaque coloured rock into a coloured powder, that powder will have good opacity too. Using that powder as a pigment to colour paint will give you a paint that hides an underlying colour well. So, certain pigments in the paint tinting machine (namely the ones that are white, mustard yellow, reddish brown, chocolate brown, very very dark brown, and black) are more colourfast and opaque than the other pigments, and using a tint formula that calls for these colours will result in your getting a higher hiding paint that fades much less due to exposure to the UV light from the Sun.

This web site won't allow a post of more than 10,000 characters (what a drag), so I broke it up into two posts. Continue on to the next post to learn about the three different kinds of plastics that latex paints are made from.

Nestor_Kelebay 01-17-2009 11:17 PM


There are really three different kinds of "oil based" coatings and three different kinds of latex paints;

Oil Based:
A) True oil based coatings: These are oil based finishes that are based on drying oils. Some naturally occuring oils have enough unsaturated sites in their fatty acids to transform into solids when exposed to oxygen in the air. These "drying" oils include linseed oil, Tung oil, Safflower oil, oiticia oil, some fish oils and "tall oil", which is a by-product of the pulp and paper industry. These drying oils are used to form coatings that transform from a liquid to a solid when exposed to air, and all of these oils form a solid film through a process called "auto-oxidation" whereby oxygen molecules in the air react whenever two unsaturated sites in the oil are in close proximity. The O2 molecules breaks apart and forms a pair of C-O-C crosslinks between those unsaturated sites in close proximity. As more and more crosslinks form, the oil molecules are less and less able to move indepenantly of one another, and the liquid oil gradually transforms into a solid. It's the formation of the C-O-C crosslinks between oil molecules, linking all the oil molecules together that turns the liquid into a solid.

B) Alkyd based coatings: These are best thought of a "clumps" of "souped up" drying oil fatty acids held together with synthetic stuff.
Since the "drying" of oil occurs when O2 from the air reacts at places where there are two unsaturated sites in close proximity, we can make oils that dry faster to a harder film by chemically modifiying those oils to have way more unsaturated sites on their fatty acids. Inexpensive oils like soy bean oil or corn oil are chemically modified so that they have a large number of unsaturated sites on their fatty acids. These fatty acids are removed and combined with other chemicals (one of which is Glycerine) to form "clumps" of these souped up fatty acids. Each such "clump" is an alkyd resin. The alkyd resins are suspended in mineral spirits, and that is the basis of "alkyd paint". When that solution is spread on a surface, the mineral spirits evaporates, thereby exposing the alkyd resins to the air. The oxygen in the air then rapidly reacts at all the spots where two unsaturated sites are in close proximity, and the result is a liquid that transforms into a much more highly crosslinked solid very rapidly. This is why alkyd paints dry to form a much harder film in a few hours than linseed oil based paints would form in a few days.

C) Urethane Modified Alkyd resins, or "polyurethanes": These are best thought of as ordinary alkyd resins that have roll cages in them, just like race cars.
An "isocyanate" is anything with a -N=C=O group in it. A hydroxyl group is a hydrogen atom bonded to an oxygen atom, like this: -OH.
When an isocyanate group encounters an hydroxyl group, they react as follow to form a urethane group, like this:

A-N=C=O plus HO-B equals A-(NH)-(C=O)-O-B

And that ugly thing at the end is a urethane group linking A and B.

To make polyurethane, you simply add di and tri isocyanates when making the alkyd resins. Since those alkyd resins are made with glycerine, and glycerine contains three hydroxyl groups, then the isocyanates you add form urethane linkages within the alkyd resin. Urethane linkages are very strong, and their formation within the alkyd resin makes the alkyd resin harder (if you tried to crush one) and stronger (if you tried to stretch one). The result is an alkyd resin that's harder and stronger because of the roll cage of urethane linkages within it. Since the formation of the urethane linkages within the alkyd resin plays no part in that auto-oxidation process by which the polyurethane forms a film, polyurethanes have the same drying time as regular alkyd based paints.

The Bayer Company (the Aspirin people) were the first ones to patent a "urethane modified alkyd" resin in 1956, and it quickly became the clear coat of choice over wood because of it's faster drying, harder and more protective film.

Latex paints:

Latex paints all consist of a mixture of solid clear, transluscent or coloured particles (called pigments) and clear hard plastic particles (called "resins") suspended as a slurry in a solution of water and a water soluble solvent called a "coalescing agent" or "coalescing solvent".

Whereas oil based coatings transform from a liquid to a solid film by reacting with the oxygen in the air to form crosslinks, latex paints form films through an entirely different process called "coalescence". Coalescence is a much less "fool proof" film formation mechanism, so there are very many more ways that weather conditions can interfere with the film formation process in latex paints.

When the latex paint is applied, the first thing that happens is that the water evaporates from the paint film. As a result, the hard clear plastic resins find themselves surrounded by the coalescing solvent at an ever increasing concentration. The coalescing solvent then "dissolves" (kinda) the plastic resins, making them very soft. The same forces of capillary pressure and surface tension that cause small drops of water to form large drops of rain in a clowd then take over and cause each soft plastic resin to stick to and pull on it's neighbors so that the separate soft resins fuse together into a continuous soft film, with the pigments suspended within it very much like raisins in raisin bread. Then, the coalescing solvent evaporates from the paint film, and as it does the plastic resins return to their original hardness. It's the evaporation of the coalescing solvent that gives you that "freshly painted smell" and signals the completion of the film formation process.

This web site won't allow a post of more than 10,000 characters (what a drag), so I broke it up into two posts. Continue on to the next post to learn about the three different kinds of plastics that latex paints are made from.

Nestor_Kelebay 01-18-2009 12:29 AM

There are three kinds of plastic resins used to make latex paints in North America:

1. Polyvinyl Acetate (or "PVA"), which you probably know better as "white wood glue". In the paint industry, paints that use polyvinyl acetate as the binder resin are known as "vinyl acrylic paints". PVA really isn't a very good plastic to make paints out of because it behaves too much like white wood glue; it remains slightly sticky even when dry, it softens up if exposed to water, moisture or even high humidity, it dries to a relatively soft film and it has lower UV, acid and alkaline resistance to other plastics. However, budget priced interior and exterior paints will use polyvinyl acetate as the binder resin because of it's low cost. General purpose primers will also use PVA as the binder resin because none of the problems that make it a poor choice in paints applies to primers; since the primer will be covered by paint, it won't be touched so stickiness doesn't matter, it won't get wet, it won't be exposed to the Sun, acids or alkalis because it'll be protected by the top coat of paint.

2. Polymethyl methacrylate (or PMMA) which you probably know better as "Plexiglas". PMMA resins are better at everything when it comes to making paint than PVA except at being cheap to buy. PMMA resins, on average, cost about twice as much as PVA resins, and this accounts for much of the cost difference between "value priced" paints and better quality paints. Paints made using PMMA resins are known in the industry as "100% Acrylic" paints. So, if you see that wording on a can of paint, it merely means that the binder resins in the can are made of very tiny pieces of hard plexiglass. There are very many different kinds of PMMA resins used to make paint, so seeing "100% Acrylic" on a can of paint is no guarantee of quality. However, it generally means that the paint will perform better than "budget priced" paints. PMMA resins are used to make everything from paint, floor "wax" and grout sealers to nail polish and other coatings. The term "acrylic" simply means "made out of polymethyl methacrylate". So, "acrylic" paints are one kind of paint that artist's use, but they are also one kind of latex paint; the most common kind of latex paint, actually. 100% Acrylic paints adhere to moist surfaces better, their properties don't change as a result of exposure to water, moisture or high humidity, they are more UV, alkali and acid resistant, they generally dry to a harder film than PVA resins, they don't stay slightly sticky after they dry, and they generally have better adhesion to most surfaces than PVA paints.

3. Styrenated Acrylic - This is the least common kind of paint in North America, but is much more commonly used in Europe. Styrenated Acrylic paints can be made to have very high gloss and good hardness, but they have poor UV resistance and they yellow with age.

There, now you know more about latex and oil based paint chemistry than most people working in paint stores.

Stay tuned to this channel and you'll learn more about the pigments used to colour paints. The colourants used in paint tinting machines consist of different coloured pigments suspended in glycerine. They use glycerine as the carrier fluid because it's equally soluble in both water and mineral spirits, so that the same colourants can be used in a single paint tinting machine to tint both latex and oil based paints. However, not all pigments are the same. They vary considerably in their hiding ability and colourfastness. Suffice it to say for now that the "inorganic pigments" or "Earth Tones" are equal in every respect to the pulverized rocks that artists have used for millenia to colour their paints, and they still provide for the greatest colourfastness and hide in either oil or latex paints. Pigment particles don't "clump together" nearly as much in oil based paints as they do in latex paints, so you can add more pigment particles to oil based paints than you can to latex paints.

So, if possible, try to find a pre-tinted mustard yellow, reddish brown or chocolate brown exterior flat alkyd paint for your house, or any colour that has a tint formula calling for only inorganic pigments. Maybe phone the manufacturer's 1-800 customer service number and confirm that the pigment in the paint is yellow oxide, red oxide or brown oxide, respectively just to make sure that you're getting inorganic pigment in the paint, and not a mix of organic pigments that would make a more vibrant mustard yellow, reddish brown or chocolate brown.

poppameth 01-18-2009 11:30 AM

Short answer: I'd coat it with X-I-M Peelbond primer to help ensure the remaining paint is glued down tight. Then top coat with a quality acrylic finish.

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