dankreboot

Here is the situation. I just painted my living room a dark brown color using semi-gloss paint. The room gets a lot of natural light so it reflects all the paint roller strokes and the reflection of light on that color is just horrible. I want to paint over it with the same color using flat paint. Is there going to be a problem painting over the semi-gloss with paint using the same color. I've already used primer before the semi-gloss and I really don't have the time to primer again. Please someone tell me it is just fine to paint a flat over a semi-gloss!!!

-Dank

mark942

OK Dankreboot.Paint over it.........................

I would give it a quick scuff.And if it were to be on another level I would scuff/prime then paint.

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slickshift

Depending on how just, "just", is. you should be fine with a nice scuff sanding and wipe down (dusting)

I'd suggest stepping up in paint quality and roller sleeve quality, and keeping an eye on your technique
You shouldn't be getting roller marks even with a dark, shiny, paint

As with any sheen of paint, remember to keep your last rolls all up and down
Flat, it's not so important, but as you're having issues, I'd do it anyway

sirwired

What specific product are you using?

Also, the most common cause of roller marks is squeezing the roller against the wall in an attempt to wring paint out of it. Instead, use a quality cover (a Purdy White Dove is a fairly cheap and safe choice) and try and cover no more than a single floor-to-ceiling stripe with one roller load. Let the roller do the work of applying the paint... all you need to do is move it up and down the wall.

Lastly, if you really want to reduce the sheen, also consider an Eggshell/Satin.

SirWired

Nestor_Kelebay

Dankreboot:

To scuff up your walls, scrub them with one of those green 3M Scotchbrite pads which are sold in grocery stores for cleaning pots.

I'm concerned that you're over-reacting to the roller marks. If you go all the way to a flat paint, you're going to discover that flat paints are harder to clean because of their rough surface, and then you'll be upset that you have to repaint yet again.

Why not opt for a satin or low-luster, and make sure that your final rolling strokes are up and down to get rid of the paint roller problems you have.

Finally, the general rule of thumb is that you prime over bare surfaces, like bare wood, bare metal, bare plaster or bare drywall. You paint over paint. So, if someone told you that you needed to prime over your semi-gloss paint before you could paint with flat paint, then you need to find someone else who knows more about paint to advise you.

mark942

quote=mark942;136044]OK Dankreboot.Paint over it.........................

I would give it a quick scuff.And if it were to be on another level I would scuff/prime then paint. [/quote]

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Nestor_Kelebay

Mark942:

The purpose of scuffing up the surface of the semi-gloss paint is to make it rough so that the next coat will stick better.

Paint and primer are going to stick equally well to that roughened surface.

So, what is the point of priming after roughening the surface of the semigloss paint? The primer won't stick any better or worse to that roughened semi-gloss surface than paint will.

slickshift

Primer absolutely sticks to a roughed up semi-gloss better than paint would
That's it's job, so it better

In this particular case, the paint is probably not even cured yet, and it's in a living room, so I'd suggest one could forgo the primer
I would not argue if someone wanted to...that would be a little extra insurance and not an extreme overkill

If this were, for example, in a bathroom, I would not suggest doing it w/o priming specifically because primer will stick better than paint
The environment is a bathroom is pretty tough on coatings...and on re-coating

Nestor_Kelebay

Primer resins have better adhesion to bare surfaces than paint resins do. However, you're not taking into account that an old latex paint is going to react to the coalescing solvents in the new paint, thereby promoting much better adhesion between paint and paint than between paint and wood or paint and drywall. That is, a latex paint substrate behaves completely differently when painted over with latex paint than other substrates do.

Latex paints and primers consist of a slurry of clear hard plastic particles (called resins) and white, clear or coloured "pigments" suspended in a solution of water and a water soluble solvent called a "coalescing agent". When the paint or primer is applied to the wall, the water evaporates, and each of those hard plastic particles finds itself surrounded by the coalescing solvent at steadily increasing concentration. The coalescing solvent "dissolves" (kinda) those hard plastic resins and makes them much softer and stickier so that not only do the resins stick to each other, they actually pull on one-another hard enough to distort each other completely out of shape. The result is that the dry latex paint film isn't porous like cooked rice, but solid, like a pane of glass.

Take a look at this web page from Dow:
http://www.dow.com/ucarlatex/coating...chive/0311.htm

where it says:
"A coalescent is often used in water-based systems as a fugitive plasticizer to soften the resin particles, enabling them to fuse into a continuous film. During the drying process, most or all of the coalescent evaporates, allowing the film to achieve the desired hardness."

If you paint over paint, the coalescing solvents in the new paint will also dissolve (kinda) the surface of the old paint to promote good adhesion between the two films.

That doesn't happen if you paint over bare wood or drywall or joint compound or an oil based paint. That's why it's generally not necessary to prime over old latex paint before repainting with new latex paints.

The purpose of scuffing the surface of the old paint is to increase the surface AREA between the old and new paint, thereby increasing adhesion. If you look at eggshell paint, it's surface is not sharp and craggy but undulating. That is, any latex paint will stick well to eggshell latex paint (even without scuffing or using a primer) simply because the eggshell paint has a larger surface area per square foot than a semi-gloss or gloss paint.

It's the coalescing solvent in the new paint that results in good adhesion between coats of latex paint, and roughening the surface of semi-gloss and gloss paints improves adhesion by increasing the surface area between the new and old paints; not by creating "keyways" for the new paint to hold on to.

If latex primer bonded better to old latex paint than new latex paint, everyone would be recommending a coat of primer before repainting, even if you're repainting the wall 10 years later with exactly the same paint. I think if you do a web search, you won't find anyone recommending that.

Can you point to any credible source on the internet that suggests it's advisable to prime over a gloss latex paint before applying a flat latex paint?

The reason why you won't find that advice is because the coalescing solvents in the new paint ensure a good bond between the new paint and the old paint. You get a still better bond if you increase the surface area between the two films.

However, if you've already scuffed up the surface to increase the surface area, you're going to get as good adhesion of the latex paint to that surface as you're going to get. Priming over that scuffed surface and then applying at least one coat of latex paint over the primer is just a waste of time, effort and primer.

Absolutely not.

Bathroom paints have mildewcides in them. These mildewcides need to be mobile in the paint film so that they can migrate to the surface of the paint when it gets wet to kill the mildew spores that land on the paint. Otherwise mildew would grow on bathroom paints in a bathroom just like it grows on regular paints.

If you paint the same company's bathroom paint over old bathroom paint, then you're increasing your level of protection because the mildewcides in the old paint can migrate into the new bathroom paint, effectively giving you a larger reserve of mildewcide to draw upon.

If you prime over a bathroom paint before repainting with another bathroom paint, you could very well trap the mildewcides in the old paint behind the primer. At the very least, you're greatly increasing the distance the mildewcides in the old paint have to migrate to help the new paint remain mildew free.

Take a look at this paper:
http://www.pcimag.com/Articles/Featu...100000f932a8c0____

where it says: "The data shows that the leaching kinetics can vary dramatically for different molecules when the paint film is exposed to moisture. The overall trend suggests that aqueous solubility of the mildewcide is a major determinant of leaching kinetics."

Which, upon translation, reads "Different kinds of mildewcides leach out of a paint film at different rates. It's the solubility of the mildewcide in water that largely determines how quickly it leaches out."

Figure 6 also shows that the porosity of the paint is also important. That's cuz the more porous the paint, the greater surface area the mildewcide will leach out across when the paint is wet.

In Figure 6, the flat paints will have too much mildewcide leaching out too quickly, thereby rendering them non-mildew resistant sooner. The gloss paint won't allow the mildewcide to leach out fast enough, so it won't be mildew-resistant from the get go. The semi-gloss paints allow the mildewcide to leach out at the slowest rate to remain 100% effective in keeping the surface of the paint free of living mildew spores. So, with this mildewcide and this gloss, the paint will remain mildew resistant for the longest period of time. Which is the optimum rate of leaching.

slickshift

You are operating under a few misconceptions
One is, if it's on the internet, it must be true
Another is, if its not on the internet, it must be false

Neither is true

I truly don't care what you find doing a "web search"
I don't care what "everyone" in the DIY world is suggesting or not suggesting
(frankly, you'll never get "everyone" to agree on anything)
I'm not here to help spread any internet myths, rumors, and falsehoods, of which the web is full of...especially in the DIY painting arena
Every day I deal with people that have seen ridiculous stuff on the web, and for some reason, think it's gospel
It came from their computer I guess...it sounds smart
Or the magazine or TV show...it must be true
Few people stop to realize it's to sell ads and entertain people
Some of the "tips" or "advice" can be absolutely scary

Anyone with a search engine can find tons of opinions labeled as facts on the interweb, and spout them till they get tunnel carpel to give the appearance of real and actual hands on knowledge

I'm here to give real advice, from actual learning and real experience, as a professional in the field

Anyway....
In my experience on the web, and with real people, you will find loads of persons recommending priming before recoating
Most of the time, it's ridiculous
Most of the time, there is absolutely no reason to prime a re-paint

I agree with you, and said as much
There is no reason to use a primer on most ten year old repaints

As I mentioned, I'm not here to give my search engine a workout and find, or not find, any "facts" to support my position on the interweb
As it costs money to host, absolutely everything on the internet has a sponsor, and I'm sure one could find lots of official looking stuff to support just about any position

You will, however, find countless professional coatings application technicians, coatings manufacturers, and professional coatings providers recommending primers for many reasons, in many situations

One of which is a wall coating over a semi-gloss wall in a kitchen or bath
Most quality professionals would never do anything but
We have seen what happens when you don't

That's not to say it will always fail
That's not the case
It's more a matter of stacking the deck
We don't want it to fail...we can't take a 25%...a 15%...a 5%...a 2% chance
It's our name, it's our word, it's our warranty, it's our reputation

That may be true
In most cases I wouldn't prime either
But in a bathroom or kitchen, all bets are off

Uh....yeah...I'll take my chances with the "fresh" mildecides thank you
I really wouldn't worry about trapping any old mildecide behind the primer

No thank you
The mildecide transfer is not the important issue
Most of the time it's not even the same paint being used, certainly not the same mix of chemicals...I can't imagine they would be compatible...I can't imagine simply assuming a quality mildecide is still functioning in the old coating...
I think you are focusing on the minutia and obfuscating the real issue

In reality, I have seen and repaired way too many failed bathroom re-paints that would have not failed had they simply gone the extra step and primed
That's why I (and most quality painters), always prime a semi-gloss bath before repainting

I hope this clears things up regarding my position and reasons for it

I'm sure the OP will decide merely to scuff and repaint

Nestor_Kelebay

Slickshift:

I'm sorry if I offended you. That was not my intent.

I only wanted to explain why painting over paint couldn't be considered the same thing as priming over wood or drywall.

You bring a great deal of hands-on experience to this forum, and that deserves respect. Not everyone is willing to share their valuable experience, whether good or bad, and the insight gained from it, with perfect strangers.

I believe there should be room in this forum for both the knowledge gained from personal experience, and the knowledge gained from technical sources. Both are important.

After all, no one ever comes into these forums with the malicious intent of giving bad advice. We all donate our time and knowledge freely so that we can be of help.