plaster vs hydrocal

[Mr.F]

i just bought rubber forms for rocks along with a carton of plaster mix.  i didn't realize until after driving an hour to get home.  then i realized that i was supposed to get hydrocal for the forms.  is it possible to use the plaster, maybe in a thinner mixture?  i have never done this before and i do not want to waste any products.  thanks for any advice.

[dgwinup]

Sure!  Go ahead and use the plaster.  While plaster of paris isn't quite as hard as hydrocal, it works well for rock molds.  Hobby-sized tubs are available at Wal-Mart.  Pick up some water-based tubes of paint while you're there.  The paint can be used to tint the plaster as you mix it.  I usually use a brown or black to tint my rock moldings.  If the molding gets chipped on the layout, it won't show a big bright white spot!

Have fun, and remember, drop cloths are your best friend!  LOL

Darrell, quiet...for now

[tomikawaTT]

If you bought the plaster mix at your (not so) LHS, it is probably molding plaster, aka plaster of paris.  Using it in your molds will be almost the same as using Hydrocal, but the rocks will be softer and somewhat more fragile.  The other difference is that your plaster will wick up more water than hydrocal would when it comes time to add more moldings or other scenic materials.

If you're happy with the results of this batch, you can buy plaster of paris at your local home improvement center - big bag, not too expensive.  Hydrocal is much less common, but not unknown.

Chuck (modeling Central Japan in September, 1964)

[jrbernier]

Darrell,

  I use regular Plaster of Paris in all of my Woodland Scenics rock molds.  The 'rocks' take stains much better than the super hard Hydrocal.  I use Hydrocal to 'paint' my medical 'plaster cloth' hills so that they are very solid.  I then add my rocks and fill in with a sloppy mix of 90 minute sheetrock plaster.  It as a working time of about 60 minutes and dries to a 'sandable' finish.  It takes stains very well, and you can even mix cheap acrlyic paints(Walmart or Hobby Lobby) into the 'soup' to pre- tint it.  I have been using this as an alternative to 'ground goop' for my scenery.

Jim Bernier

[loathar]

I always use plain old Plaster O Paris. Mix the plaster into the water. (NOT the water into a bowl of plaster powder) This will cut down on any air bubbles in your molds. I make it a little thinner than pancake batter. A teaspoon of vinegar will help to slow the curing if you need to.

[MisterBeasley]

Has anyone done side-by-side comparison tests of the various casting materials?  I'd be interested in seeing if one or the other produces a more detailed casting.  I've always used Hydrocal for my castings, so I don't have anything to compare it against.

Thanks for any info.

[Mr.F]

i just did my first batch with the woodland scenic molding plaster.  it was brownish colored and very gritty--not at all like plaster of paris.  i did the first batch with 4 ounces of water instead of three and added a little vinegar as rec. above.  the second batch was water only, and boy did it stiffen quickly.  the stuff was not at all moldable.  i had some left after filling the mold and i tried to do something with it and it was extremely crumbly.  glad i was doing rock stuff--i just mashed it into some joints to roughen the look a little.  now the wait to see how this stuff turns out.  i am suspicious that i will not get all the details of the mold wince the stuff was so gritty.

[MisterBeasley]

Are you talking about their Mold-a-Scene plaster?  I suspect that's more like Sculptamold, or Gypsolite.  It's really not made for castings at all, so the detail level you will get will not be very good.

[DeadheadGreg]

i don't mean to threadjack, but its related to the topic:  I bought the Mountain Valley scenery kit like, 10 years ago, and I found the half-used carton of hydrocal powder.  Do you guys think its still useable?

[MisterBeasley]

If it's been sealed off pretty well from moisture, it should be fine.

Why not just do a small test casting and find out?  Keep us posted.

[Mr.F]

FYI:  WalMart no longer sells plaster of paris.  Hobby stuff/crafts are basically eliminated.  Go ahead and make the trip to Lowe's!

[jrbernier]

  That depends on your location.  Both of the Walmart Super Centers in my town have Plaster of Paris.  Something like $4.97 for a 5 lb pail.  Also the Lowes/Home Depot/Menards have Sheetrock brand patching plaster(20-200 minute drying time variations) in 18 lb bags.  This stuff sells for about $7 to $11/bag, depending where you get it.  It mixes like Plaster of Paris, and you can get different drying times.  I have been using the 90 minute stuff; you get about 60 minutes of 'working' time, and it is 'sandable'.  I mix cheap acrylic paints(Walmart/Hobby Lobby/Michaels) in it to make 'ground goop'.

Jim Bernier