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PaoloM 10-16-2008 12:58 PM

Ext Paint for Interior Application?
Here's my situation...I have a rough cottage that has no furnance, plumbing, electrical, etc... the inside is finished with Plywood. I want to paint the ceiling plywood white to reflect as much light as possible, the walls and floors will be the same color (greyish color).

Should I do 2/3 coats of Exterior Latex Paint on the inside or should I use an Interior Latex Paint? For the floors I was going to use an Oil Based Solid Color Deck Stain so that it treats the wood because we will be walking inside with wet shoes/boots.

I want the floors to be very durable and the walls have to be quite durable as well and washable (if possible). What do you recommend for the ceiling, walls and floors?


Matthewt1970 10-16-2008 01:32 PM

All bare wood, plywood included should be primed with an oil based primer. Here's what I would do in your situation. Get however much oil based primer you think you will need. You will probably need 2 gallons. Also grab yourself a small quart of Rustoleum Black oil paint. Flat or gloss doesn't matter as you won't be using enough of it to change the sheen much. When you have primed your ceiling white, now tint the remainder of the primer with the Rustoleum till you have the same color (or just close) that the walls will be and then prime the walls. Now you will probably just need one coat of paint. Being that your surfaces will still be exposed to the cold and heat and humidity, go with exterior paint. Latex is fine, and my reccomendation would be Benjamin Moore. If you want light reflection and durability, go with a glossier finish. The only problem is your darker walls are still going to absorb light.

PaoloM 10-16-2008 02:15 PM

So I need to Prime for sure? So then I can get away with 1 Prime coat and probably 1 Finish coat on the ceiling and walls. Do I have to use Oil Primer or can I use Latex Primer if I'm going to use Latex Exterior Paint? If I use Oil Primer then I have to use Oil Exterior Paint dont I?


Bubbagump 10-16-2008 02:40 PM

Read what Matthew said. Yes, you have to prime... otherwise the plywood will absolutely drink your paint and your paint job will be miserable. Also as Matthew stated, use an oil based primer. Then paint exterior latex over that. As for washable.. more gloss means more washablity in general. A semi gloss ought to do you well. As for the floors, you may consider a floor/porch paint or latex enamel (real enamel, not Behr enamel) as it will likely not stain and clean up better than a deck stain assuming you are going to be walking on this surface with cruddy shoes all the time.

Nestor_Kelebay 10-16-2008 09:03 PM

If it was me, I would paint all the walls and ceilings with a bathroom paint like Zinsser's PermaWhite Bathroom paint. I'd paint the walls with the semigloss version and the ceiling with the satin version. Don't tint the paint.

If it were me, I would install a 100% Olefin carpet on the floor. It doesn't have to be installed well, you could simply cut it to fit your room, and then staple it down around the perimeter. 100% Olefin means the carpet is made completely out of polypropylene, and polypropylene is the most water resistant fiber used to make carpet out of. That also makes it the most naturally resistant to water based stains as well.

I would, however, paint that plywood floor with a high gloss exterior paint or perhaps Zinssers PermaWhite Bathroom paint before installing carpet over it so that water and mud that gets tracked in won't be absorbed into the underlying wood, but will remain on the paint surface until it evaporates.

Nestor_Kelebay 10-17-2008 11:23 PM

Actually, I've given this some thought.

There's probably not going to be any fungi or mildew growing inside that cabin if you're not using it during the winter. If you use it during the winter, there will be humidity (both from breathing, and from the H2O produced from whatever heater you have in there to keep you warm. So, if you're going to be in there in the winter, I'd stick with the Bathroom Paint, but if it's only for warm weather use, I'd just go with any interior latex paint.

Also, I'm thinking a high gloss exterior oil based paint on the floor would stand up to moisture better than the latex bathroom paint I was suggesting (covered by the carpet).

Since there's no electricity, the carpet was a lousy idea cuz you can't clean it. I'd be most inclined to probably just use an exterior oil based paint on the floor or modify your original plan and simply paint the plywood with a clear or green wood preservative, zinc or copper naphthalene respectively, wait for the solvents to evaporate from the wood preservative and then apply a deck sealer. I think deck sealers largely protect the wood from the UV light from the Sun, but they also prevent water penetration into the wood. I know little about deck sealers, but I don't see why you couldn't apply a wood preservative, wait for the solvent to evaporate so that you have dry wood again, and then apply a deck sealer over that dry wood to prevent water penetration into it.

My understanding is that copper naphthalene (the green stuff) is more effective at preventing wood rot than zinc naphthalene (the clear stuff). And, the highest copper content I've seen is in end cut preservatives. Also, since wood absorbs these preservatives just like stain, I'd probably apply the wood preservative, allow to dry completely, then apply the wood preservative again, allow to dry completely, and keep doing that until I saw no change in the "green-ness" of the wood. That way I know I've stuffed as much copper naphthalene into that would as I can to protect it as well as possible.

Sorry about the carpet idea. I was only thinking that it was water resistant. I wasn't thinking of the more practical considerations in cleaning it.

Matthewt1970 10-18-2008 12:12 AM

You don't want to use interior bathroom paint outside and you certainly don't want to use it as a first coat on bare wood. It states it is a self primer, but that really only applies to drywall. You are not climate controlling your cottage so your shed will be exposed to the exact same exterior conditions as the side of your house, it just won't get directly rained on. It will get the same extreme fluctuations from dry humidity and cold in the winter to the high humidity heat in the summer which is what exterior paint is designed to withstand.

Nestor_Kelebay 10-18-2008 01:27 AM


It's not large changes in temperature and humidity that exterior paints are designed to withstand. Interior latex and oil based paints can withstand huge temperature and humidity changes so there would be no reason to design exterior versions of these paints to cope with outdoor conditions. Your typical latex bathroom paint has to withstand the same large changes in both temperature and humidity that any exterior latex paints have to withstand. Ditto for interior oil based paints since people often use oil based paint on bathroom walls.

In the case of latex paints, the only differences between interior and exterior paints is that exterior paints will most often use a less expensive binder that dries to a softer film, will have UV blockers and more biocides added to them, and will often use zinc oxide as the white pigment instead of titanium dioxide.

In the case of oil based paints, exterior paints will be designed to dry to a softer film so that they can stretch and shrink with the swelling and shrinking of wood that occurs outdoors due to seasonal changes in humidity. Interior oil based paints dry to a film that's too hard and rigid to have the elasticity to stretch and shrink with wood. Interior oil based paints will just crack and peel on wood outdoors. You can use an interior oil based paint outdoors if you're painting metal or concrete or stucco or anything else that doesn't swell and shrink as much as wood does. Exterior oil based paints will also contain UV blockers and more biocides, just like exterior latex paints, but this is often a mute point because in you often don't need the additives that you get. If the paint is used in a shaded area in climate where fungi and mildew don't grow on paint outdoors, then there is little benefit in having those additives in the paint.

Temperature and humidity changes don't affect paints. Both latex and oil based interior paints will withstand extreme temperature and humidity changes, so there would be no need to develop exterior versions of these paints to cope with those conditions.

But, about wood...

I've been questioning something in my own mind for several months now...

If humidity changes from summer to winter caused any deterioration in wood, why wouldn't one see the same deterioration of the wood trusses and studs inside unfinished garages?

Bare wood outdoors will deteriorate. The exterior wood fibers turn grey and come loose from exposure to UV light. The wood deteriorates and ridges develop as the softer, lighter parts of the wood deteriorate and the darker and denser parts of the wood don't. The wood splits at it's ends from getting wet and drying out many times in succession. But you don't see that on the exposed wood inside a garage. The studs in a garage wall or the members of the roof trusses don't split at the ends from large seasonal changes in humidity or temperature.

What I'm saying is that the wood studs in a garage don't deteriorate the way wood exposed to the elements does.

So, it seems to me that exposure to the Sun, and DIRECT wetting by rain are what causes deterioration of wood outdoors, not temperature and humidity changes.

I think wood does swell and shrink with large seasonal changes in humidity. But I don't think that does any harm to the wood. It's the even larger changes in moisture content between wood at the end grain and interior wood cells when wood gets wet due to rain that causes the splitting and cracking of wood when it's exposed to the elements.

Matthewt1970 10-18-2008 02:04 AM

I didn't say the wood would deteriorate the same inside as it would outside, I said it gets the same extremes with the exception of of rain. I also forgot to mention direct sunlight.

You are recommending putting an interior latex paint on bare wood. Yes, you can do it, but it will have bleeding all over the place, it won't adhere well. The bathroom paints are really nothing more than a semi-gloss interior paint with just a touch more Stain-inhibiting tannin blockers but no where near the level that a good oil based primer (or even latex for that matter) would have since all it has to do is seal drywall.

poppameth 10-18-2008 11:20 AM

You do NOT want copper nap on the floor. That stuff stinks mightily. It will continuously off gas. Lay a piece of IVC or similar brand of fiber floor in there. It's virtually impervious to moisture.

Nestor_Kelebay 10-18-2008 09:06 PM


Originally Posted by poppameth (Post 173589)
You do NOT want copper nap on the floor. That stuff stinks mightily. It will continuously off gas.

I think you're thinking of creosote. I've used both copper and zinc naphthalene many times and can't recall ever smelling anything.

Creosote stinks and keeps off gassing so it always smells.

PaoloM 10-18-2008 10:10 PM

Why use Oil based Primer vs. Latex based Primer? If I use an Oil based primer will the latex paint adhere??? I really don't want to use oil based paint other than the floors if I don't have to.

This is what I think I'm going to do...what do you think?

Ceiling and Walls
1 Coat Benjamin Moore Primer (Latex)
2 Coats Benjamin Moore Exterior Paint (Latex)

2 or 3 Coats of CIL Solid Color Oil Deck Stain

Matthewt1970 10-18-2008 11:43 PM

Latex primer just really is not that good. It is ok for interior drywall or aluminum siding, but that is about it. It adhere's about 1/2 as good as an oil would and doesn't block the bleeding common with priming bare wood very well at all. I worked for a company that insisted on Bullseye 123 Latex primer and that is about the best there is for Latex Primer, but it just really wasn't as good as an oil.

poppameth 10-19-2008 11:23 AM


Originally Posted by Nestor_Kelebay (Post 173780)
I think you're thinking of creosote. I've used both copper and zinc naphthalene many times and can't recall ever smelling anything.

Creosote stinks and keeps off gassing so it always smells.

Nope. Copper napthenate is what I'm thinking of. I have a fair bit of it in my stockroom for a local tree nursery. Not nice smelling stuff. This is oil based, the good stuff. There is a copper product on the market now that is water based. Made by Wolman I think. It's junk compared to real copper nap.

slickshift 10-19-2008 12:03 PM

Do not use exterior paints on interior surfaces
Do not mess around when your health is involved

If you can find a manufacturer that says it's OK to do so, then have at it
I know Ben Moore will NOT (except for the few int/ext paints they have)

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