The thinned latex has exactly the same total number of molecules of binder as the unthinned latex. Read the example again. Adding water does not reduce the number of binder molecules, it just increases the number of molecules of water.
Because the second portion has additional water, it is thinner and therefore more coats will need to be applied to use it all up. But once it's all used up, you've applied the same total number of binder molecules to the second board as you did to the first.
I've been looking. Haven't found one yet.
Because I want to understand WHY they aren't recommended. I like to understand why I'm doing what I'm doing. If the label on the paint can says "Do not thin", I want to know why. By knowing "why", I can determine under what circumstances it might actually be permissible, even beneficial, to thin (even though the label says "no").
Common sense is good yes. But sometimes what passes for common sense is a collection of urban legends and anecdotal experiences. That's why it's good to ask "why". I'm not saying that's true in your case. You seem to have some substantial experience.
A thought just occurred to me which might be germane to this discussion. Perhaps someone familiar with how latex paint works could comment.
My understanding is that latex paint binder consists of monomers in water solution.
When you apply a coat of latex paint, and the water dries, the monomers come out of solution and begin to bond together into polymers. This bonding action forms the strong film. Once the polymers form, they are no longer water-soluble. That's why the film is water resistant, even though the original vehicle was water.
Now, what happens when you apply a second coat of latex? Obviously the process repeats itself. BUT, in addition to bonding with EACH OTHER, do the monomers in the second coat ALSO bond EQUALLY EFFECTIVELY with the polymers in the first coat, to create one seamless film (assuming the first coat was kept clean) ? Or, is the bonding between the monomers of the second coat and the polymers of the first coat only PARTIAL, so that what you get is two SEPARATE films which are bonded together, but the bond BETWEEN the two coats is not as strong as the bond WITHIN each coat?
If the latter is true, it would explain the difference between boards one and two in the example I gave in an earlier post. The first and second boards would have the exact same total film thickness, but the first board would have fewer, thicker layers; and the second board would have more, thinner, layers. What this means in practical terms as far as the quality of the paint job is still arguable I suppose.
I don't know anthing about monomers and polymers. By your description, thinning the paint too much with water keeps the molecules from bonding because they are spread too far apart. Reason I used the cut boards (molecules of paint binder) example - you can't cover the same area with half the material, whether microscopic or macroscopic. Capiche?
The boards you refinished likely had some moisture in them, having been washed two days before. Not washed, but without impermeable finish, they would be damp from being outdoors. Boards fastened onto a deck, without being finished on all sides, would hold moisture. So, when finish is applied, sun hits the deck, the moisture expands and the paint film blisters or cracks.
Because it IS film. i.e. part of the binder. But it flows better. Anyway, to fill cracks and other small imperfections, buy a paint specifically for the job. Sherwin Williams sells PrepRite High Build Primer/Surfacer, or even Block Filler (which is meant for concrete, I don't know how well it would work on wood). Other companies probably have similar things.
For your situation the most effective procedure is to use a slow drying oil based primer thinned slightly, allowing it to dry thoroughly, then topcoating with the finish of choice.
No disagreeing with you, but where did you get this information? There is no information on the product label, and the MSDS lists no ingredients at all.
I have looked and looked, and asked many contractors and paint store gurus, and no one has been able to recommend such a product for the application at hand. Remember, this application is for exterior, horizontal, previously painted, weathered, wood, exposed to sun, rain, snow, and heavy foot traffic. If anyone knows of a specific product name and manufacturer for a product like jeffc has mentioned, please post it.
Just do what I did. I wrote to the company and asked them to please send me the private patent information that contains all their ingredients and trade secrets. They sent them to me immediately.
I guess I'm stupid. I totally understood what Jeff was saying. I do the same things when traveling by airplane, going over a bridge, or turning on the lights.
I really don't have to understand everything that makes them work, and I sure couldn't explain it to a Piled High and Deep type of person in a conversation, but then, I'm just one of those stupid nontechnical types.
This thread has a distinct odor of troll .. why don't you take your vast scientific conversation to a paint chemist at a paint company? The whole idea behind paint is to keep wood from getting split, cracked and weathered, so we yokels on ahr share our experience to try to help others. Paint products that I am familiar with say "don't thin more than 10%", "sand weathered wood", "apply to clean, dry surface", "prime bare wood", etc. Since painting is a good deal of work, the preparation being the most tedious, we haul out the brushes and tarps and get the job done before the item to be protected turns to crap. You have been offered good faith advice, but challenge everyone who replies. By the time you get around to doing the project, the house will be falling down.
You should be using Decking products, following instructions and not making up your own with the wrong product. Your past failures might be the wrong product, prep or aplication on a hot or damp surface. Get and use products designed for the job and do what they say.
That may work for some here, but others insist that a person totally understand and comprehend what the paint is doing, why, and all the ingredients of said paint.
Most decking products are stains, not paint. Unfortunately, I'm constrained to use paint since the former owner painted the entire porch and I want it to match. Only the breezeway area (about 150 square feet) and steps need refinishing.
Two years ago, I essentially followed the advice you recommended above. The result was unsuccessful, despite scrupulously following the label instructions. I think I posted the details elsewhere in this thread. The paint started to fail after one year. I still don't know why for sure, although many here have suggested possible reasons, and the oil primer seems to be the likely culprit, even though "latex over oil primer" is the conventional wisdom.
Some folks say "paint all six sides", others say "no, leave the underside unpainted so the wood can breathe". I tried both ways and they both failed (six sides on the steps, only the top in the breezeway area).
Some folks say "use oil", others say "no, oil is too brittle and moisture-impermeable and will crack and separate from the wood as the wood expands and contracts due to moisture". I used latex over oil primer, as recommended on the latex label. Perhaps the "oil is too brittle" folks are right. That's my current working hypothesis at any rate.
So this summer I tried a different approach.
One set of steps I pressure washed to blast off as much blistering paint as possible. Let it dry and sanded it, then applied the latex with no oil primer. The latex is a high quality 100% acrylic latex paint rated for exterior horizontal surfaces exposed to foot traffic and weather. On half the steps I applied the latex unthinned (as per label directions), on the other half I used thinned latex for the first coat (in order to penetrate better into hairline cracks and the spaces between the boards) and then 2 coats of unthinned.
On the other set of steps I removed all the treads and risers, and completely removed all previous coating with a planer, edger, and belt sander. I used a different approach on each of the 12 boards, including replacing some of the boards with new wood; but in all cases I coated all six sides (that's why I removed the boards). For example, on one of the boards I applied the latex directly to the wood without a primer. On another I used thinned latex as the first coat, then 2 coats of unthinned latex over that. On another I stained the board first with Cabot semi-solid deck stain, then applied latex over that (yes, I know that conventional wisdom says don't do this). On another I used an oil-based water-sealing product which claimed it was paintable, then oil-primed and latex topcoat over that (letting each dry thoroughly of course). I kept a record of how each board was prepared. In a couple of years I'll see the results. Hopefully, at least one approach will endure.
I haven't re-done the breezeway area yet, but the plan is to do it the same way as the first set of steps mentioned above (power-wash to remove blistering paint, then apply latex without oil primer). Removing the breezeway boards to paint the undersides and edges and ends is out of the question. Using oil in this area is extremely inconvenient - after power washing, the waiting period for proper drying would be a real problem. Oil requires absolutely bone-dry wood. Any moisture stops the penetration. Latex is far more forgiving in this regard.
The one question that remains unanswered, and the one that has offended and/or angered some posters to this thread, is the issue of thinning latex paint with water. I was hoping there might be an old-timer or two here who understood this issue and could shed some light:
Many latex paint labels say "do not thin". The question I was exploring is whether this "do not thin" exhortation is universally true, or whether there might be extenuating circumstances wherein in would be permissible, even beneficial, to thin just the first coat, to improve penetration into nooks and crannies and spaces between boards, as long as an unthinned second (or even third) coat of unthinned is applied.
I tried to find a suitable water-based primer for this application but was unsuccessful. I looked at MANY paint stores and home-improvement centers, and spoke with a few contractors and painter friends. I have yet to find a water-based primer that is rated for horizontal wood surfaces exposed to rain and foot traffic. One guy swore by Zinser
123, so I bought a gallon, but when I got home and read all the fine print, it categorically stated "not for use on horizontal surfaces exposed to foot traffic and water".
I've also read that you should always use a primer and topcoat from the same manufacturer to assure they are compatible. I'm not sure if this is true, or if true, why.
If anyone knows of a water-based primer designed for use on previously-painted weathered wood which has been power-washed to clean it and remove loose paint, and which is compatible with Sherwin-Williams 100% acrylic latex exterior porch and floor paint, please post.
I think I've finally found a (partial) answer to my question; I'll post it here for the benefit of anyone else who might be interested:
The major solvent in today's latex paints is water. But they also contain small amounts of organic solvents, such as 2-(2 butoxyethoxy) ethanol and trimethylpentanediol isobutyrate, which function as "coalescing solvents". These coalescing solvents play an important role in the film formation, and apparently the RATIO of organic solvent to water affects the film formation. At least that's what the technical discussion in the link below appears to be saying. SO... the reason why adding too much water could be a bad idea is NOT that there is "less binder" in the thinned latex, but rather that the thinned latex has the wrong ratio of water-to-organic-solvent... and this apparently affects the chemistry of the film formation process.
more details at this link:
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The proof is in the pudding, though, I suppose. Yesterday I blasted the breezeway area of the porch with the power washer. Large sheets and small flakes of old paint were flying everywhere. But the test patches where I had applied thinned latex to scraped-bare wood 3 days prior held fast and showed no sign at all of coming loose or wearing off. So the plan is to use thinned latex for the first coat to penetrate into hard-to-reach places (like between adjacent deck boards, and where railing posts sit on the deck boards, and hairline splits in the boards); and topcoat that twice with unthinned. I'll know by next year if this approach is better than the latex-over-oil-primer approach I used 2 years ago which failed in one year.
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