Plaster Mixture

7

77railer

Member
Dec 11, 2004
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South Carolina
Hi fellas,
Never messed with plaster before and dont wanna waste it. What is the mixture I should be using, and how hard is it to carve once it dries?



77Railer
 
Plaster gets very hard when it dries and it also hardens quickly. Those are two reasons I switch to using wallboard joint compound. I've done some rockwork on my layout using joint compound, and could go back hours later and work it.

But back to plaster. You can sand it or file it, but I think it gets too hard to carve with a knife. You can work it while it's wet, but the set time is short. When I used it, I've mixed it up until it is the consistancy of pudding, and wet the surface it's going on so it doesn't suck the water out of the plaster.
 
You can find the correct mixture of plaster and water on the box or bag of plaster-of-paris. I make my own wooden and styrofoam molds and then pour plaster-of-paris tunnel portals. I have found that the "soupier" the mix, the longer you have to work it. I would recommended following the directions of the box/bag and then add more water the next time if need be. Once I used too much water and it was very easy to pour but the set time was so long that it wasn't hard enough when I took it out of the mold and the portal broke. I don't know if this was because I was impatient or if too much water weakened the plaster. Now I mix it to the consistency of thin pancake batter and pour swifty. As for carving it, I have carved bricks and stone work on the HO portals for two or three hours after pouring it. I carve with "cheap" Harbor Frieght wood chisels and an Exacto knife. I have read that you can retard the drying time with a retarder or vinegar but have not tried that yet. My next project will be a complete three dimensional viaduct with plaster of paris.
 
XavierJ123 said:
...Once I used too much water and it was very easy to pour but the set time was so long that it wasn't hard enough when I took it out of the mold and the portal broke. I don't know if this was because I was impatient or if too much water weakened the plaster. Now I mix it to the consistency of thin pancake batter and pour swifty. As for carving it, I have carved bricks and stone work on the HO portals for two or three hours after pouring it. I carve with "cheap" Harbor Frieght wood chisels and an Exacto knife. I have read that you can retard the drying time with a retarder or vinegar but have not tried that yet. My next project will be a complete three dimensional viaduct with plaster of paris.
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You are right, the "soupier" the mix, the weaker it is. Let's face it, the more water you use, the father apart the plaster grains are and the less likely they are to bond to each other. One thing I did to get a stronger bond was to reenforce the plaster. I was casting walls and poured half way, then embedded some cloth in the plaster and finished pouring the rest.

I would imagine though, if you were going to carve it after it set. you would want a weaker solution that would be easier to work with.
 
mixture

I have used POP mixes in several ways. I have poured it into molds and came out good. My problem is when making scenery and using paper towels, I start with a fairly soupy mix, but I think the towels suck out the moisture and it gets thick faster than it sets up. Does anyone else have trhis problem? If so, how do you solve it? Does using newspaper strips make a difference from paper towels? I'm fixin' to start covering my mountain/tunnel and wonder about soap or vinegar to help keep it moist longer.

Lynn :confused:
 
Lynn,

Could be that you need it to be a bit more soupy. I've never had to use vinegar, but it's worth a try. Something else I haven't done, but might be worth looking into is to possibly spray the towel with water before soaking it in the plaster. I can remember that I didn't have too much time before the plaster started to set up, but I didn't need that much either. You are not aiming for a finished surface, just a base to allow you to work on so looking great isn't your goal.
 
Lynn here's my method for what it's worth. Instead of paper I use the square gauze patches, you get them in packages diabetic supplys for needles. Instead of POP I use a bucket of mud, the type used for drywall crack filling ect. I lay out a square of gauze, moisten it with a spray bottle and with a putty knife I coat one side of the gauze with the mud. I then place the gauze in the desired location (mud side down). This takes pretty well overnight to dry thoroughly but the working time is great and a lot less messy. Have a look at the layout page on my website. ( link in my signature)
Cheers Willis :wave:
 
ezdays said:
Plaster gets very hard when it dries and it also hardens quickly. Those are two reasons I switch to using wallboard joint compound. QUOTE]

So EZ, do you think that joint compound is a better material than POP? What about spackle? I did a couple of mountians this weekend and POP is just a pain. It's either to thick which is great for rock molds and tunnel portals or to thin which means I have to come back later with the thinker mixture and "paint" over the thinner mountians. What is your opinion?
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I'm doing major renovations/additions to my layout right now, and I just opened a fresh tub of premixed "mud". I have used both, do some preliminary layering with POP just because it sets up quickly, and finish off with the drywall compound. If you do use drywall compound it is best put on in thinner layers since it takes a lot longer to dry. I have used that to my advantage when making rock outcroppings or cuts on the sides of hills. I've used a thick layer, and went back a few hours later to do some sculpturing, and it was even soft enough the next day that I could work it some more. A 12 lb. bucket only cost a few bucks and won't dry out if you keep the lid on tight when you're not using it.

I think spackle is just like POP, you have to mix it and it drys quickly and hard.

And just think, I have all this compound available at a moment's notice for patching holes in my walls, which seems to happen way to often.:eek::eek::eek::D
 
For rockwork I use the POP for strength, cleaner and crisper edges. The Joint compound even when dry (takes a long time) is not all that strong by itself and the rocks come out of the mold with bubble holes and rounded edges, and to top it off it doesnt take stain (water diluted colors) very good. But when used with paper, gauze or other material including styrofoam it helps make a great hardshell.
Cheers Willis
 
POP and mud

Willis, I changed your method just a bit. I troweled mud on paper strips to cover my mountain and build up mounds. Tomorrow I'll use POP to coat over the top. My plans are to slop it on by hand with a plastic glove, smooth it out some, and start forming for rocks, etc. The mud worked great in that I could continuously work without fear of setting up too fast. Dried overnite and continued on the next day. Thanx for the tip on wetting the old down so it doesn't sap moisture from the new.

Lynn
 
I can't speak for when you use molds, but if I did, I'd probably want to use POP as well. I bought a rock mold when I first started my layout, but never used it. I found that I like the challenge of making rocks by hand. My main tools are a putty knife and some plastic artist's knives. Lot's of fun too.. I always start off with a base color of acrylic before doing any painting.
 
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