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Old 07-07-2008, 07:52 AM   #1
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Ready to paint the porch! Oil or latex?


After 3 weeks of work I am ready to repaint my porch. I have removed all paint, and sanded. Now I am scratching my head as to weather (get it?) to use oil based (alkyd) or latex paint over an oil based (alkyd) primer. Thoughts?

2 questions:
1. What do you recommend for paint for the T&G floor? It has expanded some gaps, but the tongues are still all engaged. I am not removing it prior to painting.

2. What do you recommend for paint for the columns and trim?

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Old 07-07-2008, 08:08 AM   #2
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Ready to paint the porch! Oil or latex?


The wood is 100 years old and still hard as a rock. Very little rot and only in one location. Guess those umpteen dozen layers of paint protected it pretty well.

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Old 07-07-2008, 08:30 AM   #3
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Ready to paint the porch! Oil or latex?


The wood is 100 years old and still hard as a rock. Very little rot and only in one location. Guess those umpteen dozen layers of paint protected it pretty well.
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Old 07-07-2008, 11:30 AM   #4
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Ready to paint the porch! Oil or latex?


Leah:

I would use an exterior alkyd primer and an exterior alkyd paint.

Because alkyd resins are so much smaller than latex paint resins, they penetrate better into porous materials like wood, and therefore have better adhesion to wood. The best thing to prime bare wood with is a linseed oil based primer because linseed oil molecules are tinier still than alkyd resins, and penetrate still more deeply. However, linseed oil based paints and primers are generally no longer available. Alkyd primers penetrate deeper into wood than latex primers, and therefore have better adhesion to wood than latex primers.

For the columns and such, you could go with a latex, but my feeling is that an exterior alkyd paint stands up better outdoors than a latex.

For the floor, I would stick with an exterior alkyd paint. This will dry to a harder film than a latex paint, and therefore stand up better to foot traffic and rough shoe leather, and not get as much dirt embedded into it underfoot like a softer latex paint would.

Last edited by Nestor_Kelebay; 07-07-2008 at 11:33 AM.
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Old 07-07-2008, 06:38 PM   #5
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Ready to paint the porch! Oil or latex?


The porch in your avatar?

If I had my druthers, I'd go with an alkyd (urethane modified) Porch & Floor paint
In your case it would act as it's own primer (thinned as per instructions) for the first coat
Top coat would seal the deal and look and last great

Columns and trim I'd hafta say premium acrylic exterior
Unless there is oil on there now, then stick with that
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Old 07-07-2008, 09:33 PM   #6
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Ready to paint the porch! Oil or latex?


I agree 100% with SlickShift that a "urethane modified alkyd" or "polyurethane" porch and floor paint would dry to much harder film that would stand up better on a working surface like a floor.

However, my understanding is that this floor is a wood floor on an exterior porch.

Wood swells and shrinks with changes in it's moisture content caused by seasonal changes in humidity. It is this continuous swelling and shrinking that requires any paint you put on wood to stretch and shrink with the wood. So, on a wood floor on an exterior porch, you need to compromise because no paint will give the hardness you need to stand up well on a floor while still being elastic enough not to crack and peel off when the wood swells and shrinks.

With oil based paints, the primary difference between interior and exterior paints is how hard a film the paint dries to. Interior alkyd paints dry to a much harder film than exterior alkyd paints because you need good hardness to stand up well on a working surface like a floor, shelf, table top, window sill, mantle, etc. And, having a hard film on the wall means that it will stand up well to hard scrubbing to remove stubborn marks.

The problem is that hardness and rigidity go hand-in-hand. A paint that dries to a hard film necessarily dries to a rigid film that doesn't have much elasticity and simply won't stretch and shrink with wood.
The result is that as the wood expands with increasing moisture content, either the bond between the wood and the primer or paint breaks, or the paint and primer develop cracks in them, with the cracks normally parallel to the wood grain (cuz wood stretches and shrinks most across it's grain). This is why an interior alkyd paint will crack and peel sooner on wood outdoors than an exterior alkyd paint.

Indoors, where the humidity is much more constant, wood is much more dimensionally stable, and an interior alkyd paint will last for a very long time. It's primarily outdoors where humidity changes are much greater and last for longer that you need a paint that will stretch and shrink with a wood substrate.

Exterior alkyd paints are ones where the paint has been formulated to dry to a softer, but more elastic, film. An exterior alkyd paint isn't as hard as an interior alkyd paint, but it'll stretch and shrink with the wood, so it'll last much longer on wood outdoors before it eventually cracks and peels.

A "urethane modified alkyd" (which simply means "polyurethane") floor paint will dry to a very hard film. It's meant to be used over a wood or concrete floor indoors. It would have very high hardness that would allow it to stand up well to foot traffic. The problem is that it would also be rigid and wouldn't have the elasticity to stretch and shrink with wood outdoors. So, it would stand up very well to foot traffic on that porch while it lasted, but it just wouldn't last very long before it started to crack and peel off the wood.

Thus, the primary difference between interior and exterior alkyd paints is in how hard (and hence how rigid) a film they form, and interior alkyd paints are made to form hard, rigid films whereas exterior alkyd paints are made to form soft elastic films. And, it's entirely because wood will swell and shrink outdoors much more than it does indoors because of seasonal changes in humidity outdoors.

Latex paints, on the other hand, are more than elastic enough to stretch and shrink with wood outdoors. So, there is no significant difference in hardness or elasticity in an interior or exterior latex paint. The primary difference between interior and exterior latex paints is that exterior latex paints will:

A) have UV blockers and more mildewcides added to them to protect both the paint and the substrate from the Sun and mildew growth, and

B) will often use zinc oxide as the white pigment rather than titanium dioxide because:

i) titanium dioxide has a catalytic effect on "chaulking" in areas where the sunlight is intense. So, a white paint will chaulk more than any other colour of paint under the same conditions if the white pigment in the paint is titanium dioxide. In fact, DuPont coats it's "Ti-Pure" titanium dioxide pigments with a special coating to reduce that catalytic effect that promotes chaulking in the presence of titanium dioxide, and

ii) zinc and copper are natural biocides, which is why they're used in wood preservatives. By using zinc oxide as the white pigment in exterior paints, the zinc helps prevent mildew and fungii growth on the paint.

Both exterior alkyd and exterior latex paints have UV blockers and mildewcides in them, but exterior alkyd paints are intentionally made to dry to a softer more elastic film, whereas interior alkyd paints are made to dry to a hard film.

You do want a hard film on a floor so that it will stand up well and won't get dirt embedded in it underfoot, so that it stays looking good for longer.

However, if the floor of this outdoor patio is wood, then you also need a coating that will stretch and shrink with the wood. A hard drying floor paint will provide great hardness, but it won't last as long on the floor as a softer one that will stretch and shrink with the wood. So, what's needed here is the hardest drying film that will still be elastic enough to stretch and shrink with the wood, and that's why I recommended and exterior alkyd paint on the floor.

Leah:
Don't even think about using a latex paint on a floor, not even a so-called "Latex Porch and Floor Enamel". No latex paint dries to a hard enough film to stand up well on a floor, not even a crosslinking acrylic "floor enamel". Dirt gets embedded in the relatively soft paint, resulting in the traffic lanes looking dirty earlier. I would stick with an exterior alkyd paint on your porch floor.

If you want to try a harder paint, you can try using an interior alkyd paint. If you want harder still, then an "urethane modified alkyd" will give you a harder film still. However, the harder the film, the less elastic it is, and the sooner it will crack if the substrate is swelling and shrinking, and wood does that cuz of seasonal changes in humidity.

PS: you don't need to know the rest

If you ever look at the fir 2X12's in a lumber yard, you'll notice that they're all split for a foot to 18 inches at each end. Similarily, if you ever look at weathered wood boards outdoors, you'll find that they're always split in several places at the ends of each board. The reason for this is that wood absorbs moisture 15 times as quickly through it's end grain as it does across it's grain. Similarily, water evaporates 15 times as fast out the end grain of wood as it does out any other part of a board. As moisture evaporates from wood, the wood cell walls shrink in thickness. Since wood cells are like long narrow straws (but closed off at the ends), there are much more cell walls across the grain of wood than along the grain. So, typically the swelling or shrinkage across the grain of wood will be 80 times as high as the swelling or shrinkage along the grain of the wood. And, it's entirely due to the fact that if the wood cells are 80 times longer than they are in diameter, there are 80 times as many cell walls every inch across the grain of wood as their are along the grain of wood.

It's the fact that the end grain dries out faster than the rest of the wood (and subsequently shrinks sooner and faster than the rest of the wood) that causes tension in the wood at the end grain. The
dry wood at the end of the board wants to shrink, but the still swollen moist wood a foot or two away from the ends doesn't want to shrink. As a result, the board splits at it's end, which is the only solution that will keep all the wood happy.

Years ago, they used to paint the ends of lumber dried in kilns. The reason for doing that was to prevent the more rapid drying at the end grain of the wood, thereby preventing the wood from splitting at it's ends. But, the reason why they stopped doing that is that it slowed the drying process and lumber mills wanted to sell as much wood as fast as they could, and drying it in the kiln was a bottleneck. So, now they don't paint the ends. They just stick the prongs of their moisture meter into the bare end grain of the wood and son of a gun, it's down to a 19 percent moisture content already! Let's ship out this batch and dry another! In reality, the end grain of the wood might have dried to a 19 percent moisture content, but the bulk of the wood is still at a much higher moisture content and needs more time to dry.

Now you know why they used to paint the end grain, and why they still should be doing it but aren't anymore. And, you know why the large pieces of lumber in the lumber yard are always split at their ends.

Last edited by Nestor_Kelebay; 07-08-2008 at 09:24 AM.
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Old 07-26-2011, 06:19 PM   #7
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Ready to paint the porch! Oil or latex?


Where do I find exterior alkyd paint? Does it come in different colors?
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Old 07-26-2011, 06:51 PM   #8
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Ready to paint the porch! Oil or latex?


Quote:
Originally Posted by isisohio10 View Post
Where do I find exterior alkyd paint? Does it come in different colors?
Try a paint store- like SherWilliams or Ben Moore. They have stuff like that.
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Old 07-26-2011, 08:29 PM   #9
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Ready to paint the porch! Oil or latex?


Whooo. Blast from the past. Porch still looks good, eh.

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