Murphy's Oil Soap is used by many PMers, but I don't know if it is available in GB.  It's used on wood furniture and horse tack.  It comes in two forms, the condensed version with the screw top, and the diluted version in the spray bottle.  Use the condensed type full-strength.  Just use a piece of sponge and spread it over the plaster. 

I don't believe you need to seal the plaster with this, as the 'raw' plaster will absorb some moisture (a good thing), but it won't get wet enough to soften the plaster (which is more difficult than people imagine).

I have used Vasoline (petroleum jelly), but ran into an issue with the paint not sticking in some places.  I probably applied it too thickly, though.

Whatever you use, use it sparingly.

Jackie mentioned the dry paper method; I've never used that.  I've used a wet-paper method, where you press one or two layers of moistened (w/o glue) paper, not sopping wet, into or onto the mold, and then start applying paper that has glue applied. 

But there can be the issue that Jackie mentioned, of glue seeping through the moist layer and attaching to the mold, but it isn't too much of a problem AS LONG AS YOU DON'T OVER-APPLY THE GLUE.  Way too many people think that more glue, or thicker glue, is better, and it isn't.  Don't dip the paper into the glue, apply it lightly with a brush, then press into the mold.

One good thing about papier mache is that it's cheap to experiment with, and easy to discard the poor results.  I work with concrete, too, and bad jobs are far more difficult to dispose of... usually a hole in the ground.

Sue

I've never used plaster molds to do PM casting, but when I do make molds (usually made from hot glue or home-made clays/doughs), I use mineral oil as my release agent.  I apply a thin layer to both halves of the mold with a paint brush.  The PM absorbs a little bit of the oil, but I've never experienced any problems because of that--paint still adheres to it fine afterwards, etc.

Oh yeah, I should also note that I don't do hollow castings with layered strips, mine are solid casts.  I mix tissue paper and glue together, to create what I like to call "Kleenex Putty", which functions like sticky clay--while that works fine for me, that's probably not terribly practical for larger projects.

Here's an example of molds/casts for spider legs, that I made back in 2010:

Last edited by Patraw (2014-08-07 16:07:40)

I make a release agent from paraffin and kerosene. that is greaseless, and won't interfere with finishing.  I apply this to plaster molds that have been sealed with dilute orange shellack.
To make the release, shave or grate a block of paraffin  & place the shavings in a glass jar. Pour in enough kerosene to cover the shavings. Place a tight lid on the jar & set it down into a container of hot water, enough hot water to reach the same level as the kerosene. Take the jar out and shake it now and then. When all of the paraffin has dissolved into the kerosene the release is ready to apply.  For a new mold, apply this while still warm, using a chip brush.  Let the mold sit awhile, 15-30 minutes, then polish out with a cloth. You may need to repeat the application if the mold absorbs the first coat.

Thanks, Sue. I use the same release agent for all of my mold-making, as well.
The paper search continues. I have been sending samples of the old papers to distributers and mills in my area, but not much luck so far.  In terms of substitutes I have been trying to work with Cosmo Blotter, and will be trying out Arches Cover and Zerkall Copperplate.  All of these papers are minimally sized, but they are far more expensive than the papers I had been using.

Thanks, Sue. I will try to catch up with this lead soon. Early on in my search I contacted Dieu Donne Papermill,
www.dieudonne.org. They teach paper-making and maintain a "Paper Library" of samples, unfortunately they
couldn't provide much help.

Yes, Jim does beautiful work!  Good proportions and finishes, a couple of the most difficult parts to get right.

I couldn't get to his home page via his link in the gallery, so you can find it here:  http://jimseffens.com/html/index.html

Sue