I have used plaster of paris on several scenery projects, and have had good results with it. My main problem every time, is that the stuff starts “setting up” before I’m even done applying it to the layout. This makes it kind of hard to get the “just right” results, because I have to mix it in VERY small batches in order to use it all before it starts to harden. Has anyone else seen this? Am I doing something wrong, or is plaster just that way.
P.S. The directions on the package say to mix 2 parts plaster mix with 1 part water, which is how I have been doing it.
If you check at the hobby store there are a couple of alternative products that do not set off so quickly. I have also found that drywall jointing compund works rather well and sets off somewhat more slowly.
Add a bit of vinegar to the mix. That is supposed to retard the setup a tad. I have found Hydrocal to be much quicker, but I was rarely caught with miuch of it still to spatula into place. Adding a small bit of water…once… will buy you another minute or two.
Try adding one half a tablespoon of water to the first mix. It may work out better, or try the vinegar, about one tablespoon substituting for the same volume of water using the stated directions.
Plaster of Paris sets up in about 10-15 minutes – which isn’t enough time to do much.
Patching plaster is much better – it’s setting time is at least double – 20-30 minutes, which gives you a much longer setting time. I find patching plaster is just about perfect for doing plaster work since it includes a retarder to slow the setting time, while Plaster of Paris doesn’t have the retarder.
I keep both Plaster of Paris and Patching plaster on hand, and sometime’s I’ll mix the two 50-50 to get an in between setting time – 8 - 15 minutes.
be careful with additives to the plaster, it may affect the strength of the finished plaster. I learned in sculpting class to add plaster to the water slowly until all the water is saturated with plaster. Then mix with your fingers breifly to check that there are no lumps. To much mixing will speed the set.
Patching plaster tends to shrink when setting so cracks result. Also in your favorite home improvement store check the palsters for different setting times on the label.
Once the first layer of plaster is on,Ilike to use Sculptamold .It has a nice oatmeal like texture,has ample working time. If it seems to sticky,wet your fingers and rework the surface to smooth it out some. This works really nice for dirt or garavel road surfaces.
Vinager. Or if I’m pouring molds, I just add more water to make it a little soupy. Fills in the details better that way.(I usually add some cheap craft paint for base color too.)
I have used both Hydrocal and plaster of Paris with good results. Setting time is a problem so you have to learn to work small batches at a time. Using patching plaster or adding vinegar sound like interesting alternatives but will they weaken the plaster? I like the fact that once it sets, plaster is almost indestructible.
I have found the plaster with the longest working time to be the veneering plasters. This plaster is sold under the name Unical, Diamond etc. It is the plaster used to block and veneer over blueboard. It is fairly inexpensive and sold in bulk bags. I recommend storing the plaster in an airtight container to help stop moisture absorbtion which greatly affects the overall properties of the cured plaster.
I perfer Hydrocal for castings. Hydrocal seems to have a greater workability when cutting, sanding or fitting the cured molds. A good trick to working w/ these molds is to wet them prior to cutting. If you want a brittle chipped break or appearance work on an abulutely dry mold. Powdered masonry dyes work excellent for coloring, they have no affect on the properties of the plaster whatsoever.
Bob K.
Thanks for all the info. guys. I have a huge tub of plaster of paris, so I would like to use it. I’ll try the vinegar trick. If that doesn’t work, I might be forced to get some “patching” plaster and try that. Thanks again.
Building supply stores (not the home decrap stores) carry a plaster retarder that will lengthen the working time. I can get 1-2 hours out of a mix of hydrocal with the stuff. If you need more info, let me know and I’ll dig out the package I have.
I just keep adding water and keep it soupy until I’m finished with what I’m doing.
The best plaster I’ve ever used is the red label stuff from Lowe’s - but you’d better get it right the first time because once it sets up it’s as hard as concrete…
I use Gypsolite for the skim-coat scenery work. It sets very slowly, and has a nice gritty texture so you don’t get that smooth plaster look. I add cheap brown acrylic paint to the mix so it starts with a light earth color, which saves a lot of time painting it later.