Air bubbles in Plaster of Paris

Affiliate Disclosure: We may receive a commision from some of the links and ads shown on this website (Learn More Here)

#1
I had some bad hydrocal, so thought is try plaster of Paris in my WS rock mould. Worked fine, dry enough in just over an hour to pull off the mould. It was a good cast but I had 4-5 small air bubbles that left little holes in the rock face. I'll still use the rock, but is there a good way to avoid them in future cast?
 

Larry

Long Winded Old Fart
#2
You have to stir up the plaster after you get it in the mold to make sure there's no bubbles. When you mix up the batch, sometimes if you don't stir it well it gets bubbles before you pour it. Also, you can get bubbles if it's to thick. Just some diff. things that has happened to me when I use to use Plaster of paris. Now, I only use Patching Plaster because it is easier to mix & doesn't dry before you can get it applied on your hill. It's a lot stronger than Plaster of Paris. It's a lot CHEAPER too.
 
#3
I had some bad hydrocal, so thought is try plaster of Paris in my WS rock mould. Worked fine, dry enough in just over an hour to pull off the mould. It was a good cast but I had 4-5 small air bubbles that left little holes in the rock face. I'll still use the rock, but is there a good way to avoid them in future cast?
Easy fix. Next time you pour your Plaster of Paris into the mold make sure you wet the mold first with water from a spray bottle with a drop of soap in it. This will extend the life of your molds and make it easier to get them out. When you pour into the mold and have the desired amount in the mold make sure you "Tap" the mold hard to help the bubbles to the surface. Do this for at least a minute.

Let the mold set and if you have the odd bubble void which will almost always happen (but less often if you tap your molds when wet), then just take some wet plaster on your finger and smear and press it into the void. When this dries the void will no longer exist.

Another thing you should try is hand carving the rock and forget the molds altogether. I just mix up some Plaster of Paris and pour that over a land form I have made out of ridged foam. I wait for it to start setting up and you know it is ready to carve because it crumbles off instead of just smearing. Then all you do is pick and scrape at the surface with a small flat head screw driver (my tool of choice) and end up with something like this:

View attachment 22634

The nice thing about doing it this way is that no two rock formations looks the same because you created them yourself in place and skipped the mold. I learned the technique from seeing it on Doug Foscale's 2nd scenery DVD, here:

http://foslimited.com/DVD_PREVIEW.html
 
#4
Now, I only use Patching Plaster because it is easier to mix & doesn't dry before you can get it applied on your hill. It's a lot stronger than Plaster of Paris. It's a lot CHEAPER too.

Any brand in particular that you use? Is this the same thing as drywall mud? I solved my issue, I think I may have poured it a little bit too dry before. Made it more soupy and have turned out 4 flawless rocks since doing that.
 

Larry

Long Winded Old Fart
#5
Any brand in particular that you use? Is this the same thing as drywall mud? I solved my issue, I think I may have poured it a little bit too dry before. Made it more soupy and have turned out 4 flawless rocks since doing that.
Any brand that is interior patching plaster will work. Ace Hardware is where I get mine. Not the same thing as DryWall Mud. Read the message from NW Canuck. He has a good idea for you too. You can carve Patching Plaster the same way. You can also add color to the plaster w/latex paint or water mixed powders for coloring. I use Aluminum Foil(Reynolds Wrap) to make molds too.
Crinkle it up to the size you need & pour it w/your mix. Since the foil is so thin you can double up on the layers. Before you pour in the plaster or Plaster of paris I spray it w/a mixture of Dawn dish soap & water.
There's all kinds of rock mold material. you can find a scared up rock & wrap the foil around the rock & then take it off & use it for a mold.
 



ModelRailroadForums.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com