Are you just curious?

No.

exterior paint is for exterior, regardless of similar conditions.

If you have an excessively humid bathroom, get a fan installed.

You can paint a bathroom with an interior enamel paint, but it isn't necessary, any interior paint is suitable.

Benjamin Moore makes Aura Bath and Spa paint specially formulated for higher humidity areas.

Many companies are now making interior paint specifically formulated for bathrooms and kitchens to withstand the high humidity. No need to use exterior paint.

I am thinking about repainting the shower area walls (Hardi-Plank), so that might be a tougher exposure.

I had a veteran painter tell that exterior paint should never be used inside. The additives that give it protection from the elements are not healthy.

there is no practical reason to use exterior on interior walls. you can go down the chemistry rabbit hole - valid points.

from a practical pov, however, interior formulations aren't less than exterior -- they're simply built for interior application, wear, use, durability, etc.

Are your shower walls Hardi board, not tile?

Can you post a picture of the area you want to re-paint? You might get some good suggestions.

NO it i not for the purpose you are planning . Do I understand you have hardie plank for your shower walls how did that happen?

Hardi plank applied weatherboard style like you do outside. I think boards are 6", exposure is 5".

How a bout a picture It seems like a strange choice to me .

I care not what you paint it. It's a shower fail

Use marine paint meant for a BOAT, and get a gas mask, and get out of the house for a week Or two.

Was this some diy, and you created a bath from an exterior porch? Good luck...........: (

"While HardieBacker cement board is a standard industry choice for shower walls, using Hardie shiplap siding inside a shower is generally not recommended by professionals and may void manufacturer warranties. Traditional fiber cement siding is water-resistant but not 100% waterproof. In a shower, where direct and constant water exposure occurs, using siding products can lead to moisture trapping, mold growth, or delamination if not part of a specialized, fully sealed system. Performance in Wet AreasExperts from Angi and UFP Edge advise against installing shiplap in areas frequently exposed to water, such as shower surrounds, because tile and shiplap are not interchangeable for those environments.

• Water Absorption: While James Hardie fiber cement resists swelling better than wood, the core material can still absorb moisture if cut edges or joints are not perfectly sealed.
• Maintenance Hurdles: Reviewers on Houzz note that the rough texture of fiber cement siding can collect soap scum, skin cells, and shampoo, making it difficult to clean in a shower environment.
• Installation Risks: To make it work, you would need a 100% watertight substrate (like a waterproof membrane) behind the shiplap, and any fasteners used to attach the siding could puncture that membrane, compromising the system.

Yow!

The house (including bathrooms) was designed by an architect. We have the same material on the exterior of the house. We have been in the house for 18 years.

Someone had to be at the bottom of the class : )

The EXTERIOR of your home is fully exposed to air, it is fully exposed to sunlight.....It's an outdoor material.

Just not the best idea for a shower, and if you just did it? You did.

Maybe I was not clear. This wetroom, these walls, were installed 18 years ago. I did not just do it.

The architect had to create a drawing package and get it reviewed/approved before we could get a permit.

The answer to your initial question is: No; it is not common practice.

Was this bathroom created as part of an addition to the exterior of a pre-existing house? It sure looks like it. It may have seemed like a good idea at the time, but no matter how many government officials rubber-stamped the plans, it left you with the practical, aesthetic, and safety issues you face now.

Rather than compounding 18-year-old design and construction errors by using outdoor-only paint in an indoor room, I recommend you consider gutting the bath and having it re-designed and rebuilt with materials designed for the purpose the space is used for.

After 18 years, I have to wonder whether there is a substantial amount of mold and/or mildew under those planks. I would be compelled to remove it and tile it as is recommended. I am amazed it has held up this well for this long. Apparently the original architect went too far in an artistic direction and lacked practical knowledge of the materials he/she created this look with.

There are showers that have the shiplap appearance but it was created using ALZEK exterior building planks/trim. It is PVC based and NEVER needs to be painted.

https://azekexteriors.com/products/specialty-items/shiplap?iframe=true&partnerid=

HOW WE USED SHIPLAP FOR A BATHROOM.

https://www.savvyinteriors.com/designblog/shiplapshower

Savvy Interiors demolished a classic bathroom made with antique tile in a historic building and replaced it with vinyl. À chacun son goût, mais pourquoi?

If someone removed a classic bathroom for this, they should be hung at dawn. Really, guys!? Who does stupid s—t like that!?

I would start by cleaning thoroughly with a TSP substitute to be sure there's no soap residue on the surface, paint on a mold and mildew resistant primer, and then use the Aura paint for humid areas.

Many good quality exterior acrylic latex paints are fine for interior spaces, but… I think your assumptions are incorrect. Exterior paints are typically formulated for flexibility as the exterior of your house tends to move a lot more than the interior. That helps it resist cracking and peeling in temperature swings but that doesn't mean it is ”washable.” Which would likely be a better goal for a shower wall paint.

Your best bet is likely a water based epoxy. That is what you will often see in hospital areas that are going to see a lot of moisture. SW has a coating for tile called XIM tile doc that is a water based epoxy meant for painting tiles. It should adhere fine to hardi-plank.

Alternatively, use any good quality washable paint with a mold and mildew additive. I would look at BM Advance with the M1 Mildew additive.

Good luck.

@amystoller, parce qu'ils sont fous.

dani_m08, that is some serious research. My hat is off to you.

amystoller,

Re: your question.

I thought I was clear in the posts above yours. The house, along with these bathroom walls, was designed/reviewed/permitted 2006-2007. House (complete with these walls) was built on a vacant lot 2007-2008. Hope this helps.

If you see an aesthetic issue, that's ok. It's on you. We do not see one.

Mold came from heavy slat platform on shower floor. Removed platform (not replacing), scrubbed, scraped and applied 2 coats of Kilz to bottom 3 courses. Paper you see on the floor is "dropcloth". I have paint that was color-matched to original eggshell. Think I will use that.

Soap scum/oil will be a problem on any surface.

Thank you for clarifying that the bath was built at the same time as the rest of the house. As you like the way your bathroom looks, I hope you are able to save it. If you think you can save it by using materials not suited for the job, you may be right, but those who have cautioned you against it have given you reasons why the smart money is against it. Either way, using outdoor paint indoors is not common practice.

Whatever.......: ) I think making simple, hard, is just that. Has a way of biting you in the fanny.

dani_m08,

Thank you so much for the information!!!! You opened a lot of eyes, including mine.

Re:your question

Walls were prepped somewhat like an exterior wall, with roll-on waterproofing (similar to Redguard) over bluboard.

Again… what you want is a low VOC waterbased/waterborne epoxy. It is a waterproof coating and since it is a water based paint it partially cures by evaporation and is vapor permeable. For extra protection add an antimicrobial additive for mold and mildew protection.

You could build a shower out of cardboard if it was coated in a good waterborne epoxy.

Alternatively, you can find an indoor acrylic polyurethane. It is what cars and sometimes furniture are painted with. It is a great durable coating but a bit harder to apply with extra safety restrictions. They have polyisocianates and should be left to cure 24-72 hours to ensure all the polyisocianates have reacted and are no longer breathable. That is less of a problem if you are brushing it on, but I have never seen it brushed on.

There are a lot of people getting bent about hardie plank in the shower… For most of human history we stored water in wood barrels. I suspect that Hardie plank has superior water resistance to oak so I suspect it is fine… but maybe I am wrong.

ETA: Polyisocianates are non-toxic once reacted and will likely finish reacting in a few hours. The 24-72 is pretty cautious but there is no reason not to be cautious. If you have another bathroom you can just seel off the ducts and doors with plastic.