How to mix plaster of paris correctly

[jwr_1986]

We have been attempting to use plaster of paris for scenery work but we must not be mixing it right because it seems like the water and the plaster never fully mixed. After it is dry it is very soft and powdery, it does not have any body to it. If anyone knows what we are doing wrong please help.

Thanks,
Jesse

[jfugate]

Is this fresh plaster you just purchased? Your post sounds like old plaster to me.

Plaster is very cheap ... you can get a couple pounds of it from Walmart for about $5, so I'd recommend you get some fresh stuff, if that's your problem.

[Tracklayer]

I'm no expert of plaster, but from my own personal experiences, I've found that if you mix it thick and work with it fast, the results will come out a lot better. You can always use a wet paint brush or foam bru***o help shape it while it's still wet. As for thinly mixed plaster, it works good for dipping paper towels in and laying them over a form to build a mountain. You can also use aluminum foil to make molds for rock formations by crinkling it up, then straightening it out and making it into the shape you want. Once all done, you can paint it with flat spray or brush on earth tone paints.

Hope this helps...

Tracklayer

[jwr_1986]

The plaster isn't even a year old. We were using the dipped paper towel method and it worked very poorly. The plaster just wouldn't mix with the water and it never dried hard. it only kind of stayed powdery.

Jesse

[Anonymous]

I used to make art molds with Plaster of Paris.. sounds like you have a bad bag, I'd get a new bag and try again.

[jfugate]

QUOTE: Originally posted by grandpacoyote

I used to make art molds with Plaster of Paris.. sounds like you have a bad bag, I'd get a new bag and try again.

Agreed. Plaster should easily mix with water. You mix it to a thick cake batter consistency and dip the towels in it. Hold up the towel and let the plaster drain a bit, then put the towel down where you want it, overlaping it with other towels.

In about 15-20 minutes it should get very warm and start to harden to a nice smooth and solid shell.

[Tracklayer]

QUOTE: Originally posted by jwr_1986

The plaster isn't even a year old. We were using the dipped paper towel method and it worked very poorly. The plaster just wouldn't mix with the water and it never dried hard. it only kind of stayed powdery.

Jesse

I agree with grandpacoyote. It sounds like you got a bad batch of plaster. I always try and get mine from Lowe's that's in the red and white milk carton looking container.

Good luck with your project.

Tracklayer

[selector]

I have limited experience with either P of P or Hydrocal, but what I have tells me that Hydrocal is a b***h to mix. It wants to stay on top of your water, and takes forever to get it into the water. Thereafter, it seems to want to set up in just a few minutes, even when mixed exactly as the WS boxes of the light Hydrocal state. I found that the Hydrocal would begin to thicken very quickly, even abruptly, after about five to six minutes.

As for the P of P, it mixed much better, and took tens of minutes before it began to harden. This was expecially true when mixed per instructions on the side of the box available from Walmart. That stuff was like thick creme, to my mind, AND very SOUPY. Poured into the WS rock molds, it would take at least 45 minutes before I could get it to come out in one lump.

Your experience seems to be one of two causes: improper mixture (including something odd about your water) or faulty product, mentioned already. That's the way she goes...

[nedthomas]

Try adding a drop or two of dish washing liquid to the water to help break the surface tension.

[selector]

Ned, I wish I had thought of that. But, you are most likely correct, and I'll be sure to do that next time. [:)]

[DavidGSmith]

I use PofP to cast rocks and mixed soupy for use with paper towels. No problems. It sets up quicker if you use warm water. My son is a plasterer and he calls it quick set. I will ask him when he gets home if he has ever seen your problem.
Moose

[jwr_1986]

I contacted the dealer I purchased the bag from and they in turn had me call the manufaturer, DAP. They asked how I was mixing it and basically I was doing two major no no's for thier brand. I was adding water to plaster and I was using warm water. They recommend the coldest water you can use. To the point of refrigerating tap water. They say this is because of a heat activated additive in their plaster that causes it to cure. I'll give it a shot the next time I'm down to the club. Thank you all for your thoughts I'll report back my results.

Thanks
Jesse

[bogp40]

The temperature of the water does have an affect on drying time and mixing plaster into water or visa- versa these problems never should happen no matter what they are telling you. If the plaster was old or left to absorb moisture during storage or the quality of your water are the only factors I know of that would cause you this trouble. I would recommend using a different brand.
I always use hydrocal for casings. Bought in 99# bags and once opened is placed in 5gal sealed pails. A soupy mix of hydrocal is great for the paper towel method.
For all other plastering, base coat and top layers of hard shell, I find that veneering plasters(Unical, diamond etc) work the best. This is also stored in sealed conatainers.
Many use Plaster of paris with no problems, either try a different brand from another supplier or go to HD and get a large bag of Unical. Hydrocal can be a problem to find in various parts of the country and can only be found at more specialty building suppliers. Ask a drywall board hanger or plasterer if you have trouble.
Bob K.

[Anonymous]

I've never used plaster of paris on any model railroad. I just use regular plaster or hydrocal or foam. The plaster of paris cracks too easily. It's not flexible at all.

[DavidGSmith]

As promised ealier I asked my plasterer son. He thought maybe your plaster was stored in a damp place. In other words it had already worked with water but not enough to harden or set up. Try a new container.
I use cold tap water and it still sets up pretty fast.
Good luck

[dgwinup]

QUOTE: Originally posted by jwr_1986

I contacted the dealer I purchased the bag from and they in turn had me call the manufaturer, DAP. They asked how I was mixing it and basically I was doing two major no no's for thier brand. I was adding water to plaster and I was using warm water. They recommend the coldest water you can use. To the point of refrigerating tap water. They say this is because of a heat activated additive in their plaster that causes it to cure. I'll give it a shot the next time I'm down to the club. Thank you all for your thoughts I'll report back my results.

Thanks
Jesse

Follow their instructions! It IS possible that your plaster is old. Old plaster acts just as you describe: takes a long time to set and is powdery when dry.

The same thing happens when you add water to plaster. In order to get it mixed, you end up adding too much water. Really soupy plaster will take a long time to set and will be powdery when dry. I have mixed large and small batches (Quart-sized to medicine-cup size) and have had no real problems unless the plaster was old. If DAP is also adding a heat-activated hardener, their plaster will set up too fast when mixed with hot water.

Start with your water and gently sprinkle the plaster in as you mix it. Don't just dump it in, it will clump and won't mix right. I haven't used any liquid detergent with my mixes, but that might help keep the plaster from clumping. Adding it slowly while mixing does the same thing. Keep adding and mixing plaster until you reach the consistancy that you need - a little soupy for hard-shell work, thicker for casting.

BTW, use old plastic butter/margarine containers for mixing. When you are done, let the remaining plaster harden in the container, then flex it to break out the hardened plaster. Also, try NOT to clean your containers and tools in your sink! Use a bucket to clean most of it off, then clean-rinse in the sink. Good plaster WILL harden underwater if there is enough of it. The chemical reaction of plaster hardening produces heat and does not need air to harden! Getting hardened plaster out of your drain pipes is an expensive proposition! Do NOT ask how I know!

You can also pre-color your plaster mix. Tempura paint powders are recommended, but I have used acrylic tube paints and Ceramicoat hobby paints from Wal-Mart. The darker colors work best and usually dry to a lighter shade than when wet.

You will know when you are mixing it properly because it will give off heat as it hardens. If you use hot water and a quick pour for molds, it will harden quickly, sometimes in seconds. The color tints will slow the reaction down a little, but it should still harden.

Where I live we have some hard water and I have used water from my mother's old house which was even harder. It didn't seem to make much difference to the plaster. If your water is heavily treated, especially if there is a chlorine smell, let the water stand overnight before you use it.

Plaster is a wonderful substance for model railroading. It is cheap and readily available. It bonds well to most surfaces. It works well for hard-shell scenery and castings, and has other uses. It can be messy if you are not careful.

Hope this helps. Didn't mean to go on so long.

Darrell, returning to quiet...for now

[fievel]

I add a tablespoon of cider vinegar to each 2 cups of tap water when I want to slow
the set time. Works great.

[MAbruce]

One more tip (please don't ask me how I know this[:I]).

DO NOT WASH ANY EXTRA UNHARDENED PLASTER OUT OF YOUR PAN DOWN THE DRAIN.

Let it harden and chip it away - or it will harden somewhere in your pipes! [:0]

[nedthomas]

I know vinegar slows down hydro-cal. Will have to try it with plaster of paris. Thank you.

[Anonymous]

Adding a couple of drops of liquid dishsoap makes the water "wetter" by reducing the surface tension which then allows the dry plaster to more easily mix in with the water.

Does anyone else have the problem of when you're up to your elbows in wet plaster your nose starts to itch? :)

[DavidGSmith]

Morgatai--Of course thats Murphys Law at work.

[trainwomen]

I've used huge quantities of plaster over the last two years on my layout - about 25 bags. I use casting plaster which is basically plaster of paris. I buy it in 20kg bags which makes it very cheap. I have not had any trouble with mixture, although the drying time can be a little unpredictable depending on the weather - dries quicker when the days are hot. I mix it in 4litre ice cream containers and always clean them after use by flexing as described by others. If you leave old plaster in the bottom of the container and mix with new you sometimes get a bit of a hot mix and it goes off super quick. Sometimes it suits me as I enjoy hand carving rock outcrops with a chisel as the mixture is drying. As for mixing I have added water to plaster and plaster to water - both work ok but you tend to have better control by adding plaster to water. Like many of the other contributors I would suggest that the plaster you were using was suspect and you were just unlucky. Anyway keep on trying as it is great stuff to work with.
Kind regards from Australia

[trainnut57]

I just finished using the paper towel and plaster of paris method for a tunnel and hill with great success. I must agree with some of the people, you may have a bad batch of plaster-it happens. I also learned that you cannot follow the directions for plaster-to-water ratio provided by the manufacturer. I don't know who did their research but they must have been on some real good drugs. The last box I bought said to mix 3 parts dry plaster to 1 part water-you gotta be kidding. There wasn't enough water to do anything but make a couple of "air balls". I found if you add water to get the consistancy a little wetter than that of pancake mix, you're in business. Remember, this is a very cheap way to make scenery, especially if you cannot readily purchase plaster cloth (which is comparitively expensive). It doesn't cost you that much to practice, and we all know practice makes perfect. Good Luck.
Trainnut57

[fkrall]

FWIW, an expert modeler friend advised always to buy the largest bag of plaster feasible, as it can vary slightly by lot. If you extend work by mixing a different lot, it'll vary slightly but noticeably in texture. I've not created scenery yet, but I'd take anything Tom says to the bank!