Does anyone know what happens to drywall mud, once it dries completely, when it is outdoors?
I am extending my layout outside under a covered porch where it will not get wet but will be subject to freezing temperatures and a wide range of daily temperatures, often 30 or 40 degrees in one 24-hour period, here in Colorado.
What will happen to the drywall mud/joint compound? I use it on blue/pink foam to create rock and ground formations, then paint it with a gesso/latex paint before staining/painting to look like rock and ground.
I don’t have any direct experience with drywall mud outdoors but I have seen plenty of problems indoors when it exposed to moisture. The usual result is mold. With the layout being covered by the porch I would be concerned that it would not receive enough sunlight to stay dry.
I would also be concerned with the temperature swings. I’m not sure that the mud would stay bonded to the foam over time.
As an alternative, I have seen lots of great rock formations carved straight out of foam. I believe that the foam by itself would be fine in those conditions because when used as insulation it has to withstand constant temperature changes. Exterior grade paintable caulk could be used to hide seams and it would also not suffer the moisture/expansion problems.
One other suggestion I would make is to post your question to the garden railway people on the Garden Railway forum. http://grw.trains.com/ I’m sure they can give you lots of good advice.
Hey John, how’s it going? It’s me, Michael, you printed my backdrops for me!
You’re already running out of room, that’s funny.
As far as the mud. I would think, if your sealing it with Latex you should be fine. Not sure if the Latex might crack on you though, with the cold winters and warm summers.
If you have calendar time to experiment, it would be interesting to try under your specific conditions. However, I believe it highly unlikely that it will withstand many cycles of weather because at the very least, it is likely to develop small cracks which will admit the moisture in the air and then there it goes!
Latex exterior paint can certainly withstand temperature changes and moisture exposure, but the joint compound substrate likely will not.
You could try portland cement mortar, a product designed for outdoor use, but I don’t know how well it will adhere to the foam in thick applications such as rocks (and it’s heavy). Perhaps you could use actual rocks and bind them to the foam with a thin application of mortar. Or forget the foam and use galvanized mesh as the substrate for the cement mortar.
There are also synthetic stuccos that are made to apply over extruded foam substrates (sometimes with a mesh sheet over the foam), but again, they are intended to be applied thinly.
Hiya Michael…actually I moved households and am starting over, older in the hobby and wiser I hope.
I think I’ll take Dave’s advice and post in the garden/large-scale forum; I just thought somebody here might have construction experience or something with drywall mud used in protected places outdoors.
I’m getting the idea from the limited responses that my initial trepidations about this practice might be justified, but I thought I’d ask just in case someone with experience says: “Oh yeah, no problem, go for it!”
I would agree with Critter and go with straight carved foam painted with exterior latex paint. I and others have modeled very realistic rock formations with nothing but foam. Personally, I can’t see messing with plaster, plaster cloth, drywall mud, etc. Just my preference but it would definitely be best for a freezing outdoor application.
I don’t know where in Colorado the op is located, but humidity out here can range from low, 30 to 40% to REAL low, less than 10% so I don’t think relative humidity would be a problem. However, the layout should be under a roof to protect it from the occasional rain and the harsh high altitude sun.
Drywall mud will dissolve if water or moisture gets to it. Think of a porch ceiling that has roof seepage or moisture above. I’ve seen joint tape hanging down after the mud got soft.
It’s OK as long as there is NO moisture.
I would not use mud outside. Even painted you are in for trouble.
What I did over my fireplace where I did not want anything flammable within the eighteen inch (distant from) requirement, was to use cement board. I used 1/4" screen for the cliffs. I have been doing a lot of renovations around the house and as a result I used the leftovers in buckets after doing some work on the house. I would run into the trainroom with the bucket of leftover mortar after putting down the marble floor and slop it on the cliff face. Other leftovers I used were included cement topping mix, tile mortar, tile grout and I also used sifted readymix concrete. A $4.00 bag of readymix concrete with the rocks sifted out with the wife’s kitchen sifter is a really inexpensive way to go.
All these things are inexpensive and will not be affected by temperature.
Here is what I am talking about. The part over the fireplace is unpainted. To the left you can see where it butts up to my pink foam cliffs that are painted. Once painted the two are indistinguishable.
Here are some close ups of my cement, grout, mortar and topping mix cliffs. Unpainted, thus the different colours.
One other thing you can use to make temperature resistant cliffs is aluminum foil. Here is a bad photo of a tunnel liner made out of aluminum foil. I have seen cliff faces made out of tin foil and they looked great.
I would also consider using steel studs for benchwork. Foam on top of steel studs should not cause problems as far as temperatures swings are concerned. Steel studs are inexpensive to buy.
Cement products should be fine with the foam. My basement has thick foam up against the foundation in the crawlspace.
This product is probably not a good idea for any kind of outdoor use. Unlike setting-type plasters (casting plaster, Hydrocal, etc.) it always remains soluble in water, and there just isn’t a reliable way to seal out the moisture regardless of how you finish your scenery. I’ve seen it used on the undersides of porch structures, and the results almost invariably look horrible after a few seasons of weather, with mud flaking off, tape coming loose, and so on. Take a look inside structures where there’s a water problem, and you can see drywall mud that’s been in place for decades suddenly dissolving once water gets past the paint or whatever was supposed to be protecting the mud.
It cracks, don’t use drywall mud for anything but drywall as you will end up with proublems. After all it can have proublems with any moisture used as intended but I understand the idea. I use patching plaster for those things. Only a proublem it that stays wet. I was in the trades for over 30 years.
Thanks guys; I’m convinced not to use drywall mud outside…I’m going to use all-weather caulk to fill the cracks in the foam and I’ll probably take VSmith’s suggestion to use smooth stucco or something like that for whatever else I need.
I’m also going to look very seriously into the other suggestion to use steel studs for most of my benchwork…I guess I’ll have to learn a new skill; I hope it’s not too difficult to learn and use. I see youtube in my near future.
