Casting and Painting Rocks

Best thing would be to watch some Youtube videos - https://duckduckgo.com/?q=atta...videos&ia=videos

Great idea, Joe Fauty.  Thank you for the link.

I've just recently gotten into this and I enjoy it. One thing I learned is that OLD PLASTER can be a problem. If you have any of the WS "Lightweight Hydrocal" or even just regular wall-repair type plaster, it absorbs water from the air over time. So it won't mix normally or can have problems hardening (sometimes it NEVER completely hardens). Plaster is reasonably cheap. I prefer to replace old plaster with new if it has been sitting for a year or three.

A test I like is to put a cup of COLD water in a clear container and dump 2 cups of plaster on top. You should be able to watch (through the sides of your container) the plaster absorb the water and then sink to the bottom (the process is kind of cool to watch actually...). If it floats or doesn't seem to react quickly with the water, I throw it out. But an expert probably knows how to make this right and use the old plaster. I'm a novice so I just pitch it.

Also, for the WS products that mix several kinds of plaster, as seems to be the case with Lightweight Hydrocal and several other WS products, the "heavier" plaster settles inside the box over time. The WS instructions in these cases say to shake the box for 30 seconds. I have found that a shake is not enough. I prefer to empty the box out into a container and stir it thoroughly. Then use it all. Don't let it sit until next week!

Finally, and maybe obvious, NEVER ever clean your containers and mix stuff in the sink--throw it out or wipe it out with rags and throw them out. There's a word for it that I can't recall, but once plaster hits water, it WILL HARDEN (or at least solidify to some extent), even under water! It will not dissolve. You don't want plaster hardening in your pipes.

Good luck!

Don

  USE PROPER PPE.

   Paint your rock molds with Gesso paint to act as a release agent and primer.  Use Alumilite casting resin in the molds. You don’t “fill” the molds, just enough to coat the mold and fill all the crevasses. Don’t buy (use) the small marble size molds. By the time you get all your mold’s poured ~10 the alumilite will have set on the the first ones. Remove your rocks and give them another coat or Gesso. You can use scissors to cut and trim your rocks, providing you with your own small rocks.

   Fill your rocks with expanda-foam and place in the desired place, over lapping the edge’s turning them this way and that to break up look. If the foam oozes out let it dry and trim to match. Try to use foam that doesn’t expand. You only need dark blue, yellow and orange paint to get realistic looking rocks.

  We stuck ours directly on the pink foam. However some were on plaster cloth, that foam sticks to everything. 🥴 We made a portable layout (4’x6’) that weighs a little over 20lbs.

Brad J

Thank you everyone for you suggestions.  I did buy a "new" box of WS lightweight hydrocal and the rocks crumbled after I cast them.  I thought I did something wrong, but maybe the hydrocal had absorbed moisture? 

Never heard of Gesso or aluminite for rock casting.  Where do you get those products, Brad?

Picked up the Gesso and Alumilite at hobby lobby. Look close because they don’t carry very much at a time.

Brad J

Thank you.

I have used nothing but JOINT COMPOUND THAT IS USED FOR SHEET ROCK ,& i have never had any problems covering plaster cloth ,as for rock molds i don't remember for sure but i think i used hydrocal ,hope this helps .One thing i forgot to mention is i think you may need to spray something into the mold before you put the hydrocal in ,maybe someone else can verify that ,this way it is easier to release the hydrocal .

I start with a base of crumpled newspapers which I the cover with paper mache or Scenic Express plaster cloth on the steeper areas. Then, using rocks cast from plaster in commercial rubber molds, I add rock detail where appropriate and then work it into the scenic base with plaster.