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Mixing plaster of paris

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  • Member since
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Posted by SeeYou190 on Tuesday, October 19, 2021 9:40 AM

The OP has his answer. 

Copy-and-paste answering is in full effect.

The merry-go-round is beginning.

-Kevin

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by Water Level Route on Tuesday, October 19, 2021 9:46 AM

I have always found it best when experiencing difficulties with something and not following manufacturer instructions, it's best to go back to square one and follow the directions.  Curiously, that sometimes solves the issue.

What gets added to what may not make a difference in many cases.  However, there are some where it makes a big difference, such as acid dilution.  Always add acid to water, not the other way around.  Logically shouldn't make a difference.  Reality shows otherwise.

Mike

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Posted by Water Level Route on Tuesday, October 19, 2021 9:47 AM

SeeYou190

The OP has his answer. 

Copy-and-paste answering is in full effect.

The merry-go-round is beginning.

-Kevin

 

Good point Kevin.  I will cease and desist in respect of that as well.  Thanks for pointing that out.

Mike

  • Member since
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  • From: Southern Florida Gulf Coast
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Posted by SeeYou190 on Tuesday, October 19, 2021 9:57 AM

Water Level Route
I will cease and desist in respect of that as well.  Thanks for pointing that out.

Our "smarter than everyone never wrong argues about everything" guy showed up in here, and the level of conversation took an immediate plunge.

Steve had to lock another thread yesterday after it was "Spiked" by this guy.

I am so tired of this merry-go-around in what should be the premier Model Railroading discussion forums on the internet.

-Kevin

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by rrebell on Tuesday, October 19, 2021 10:53 AM

Funny, many times the instructions are just wrong. With plasters the instructions they give are for (hopefully) never fail jobs of a larger nature, we can be much more percise to get what we need as it is ussually small batch. In other words a little more or a little less may be what works. This is because in small batches and the way we work, it may take a bit more time or less time to do a pour. This was learned by me when working in the 1-1 world.

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Posted by richhotrain on Tuesday, October 19, 2021 11:14 AM

rrebell

Funny, many times the instructions are just wrong. 

Is that really the case?  I can speak for WS Light Hydrocal. The instructions for mixing are precise and accurate.

Rich

Alton Junction

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Posted by selector on Tuesday, October 19, 2021 12:48 PM

Only one quick contribution from me, based on experience:

Yes, use soapy water to coat the inside of the mould before you pour the mixture into it.

Also, consider light jiggling, even lifting and dropping short distances (3-4 mm, tapping) to get air to lift and move up to the surface once you have the pour in place.  This is not an easy thing to do, for obvious reasons, but it can greatly reduce the friable nature of the product when the material cures.

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Posted by Lastspikemike on Tuesday, October 19, 2021 2:32 PM

The problem experienced by the OP was not likely due to mixing method. The proportions were a bit on the wet side which does provide a likely cause for the difficulties experienced.

It literally does not matter which way round you mix the correct proportions for your intended application. The final consistency of the mix gives a good indication of how it will set. For moulding (casting really) you want a drier mix than you would use for say a broken arm splint.

Often it can help reduce cracking if you introduce fiber into the plaster. Short bits work better than long threads. This is commonly done in concrete work where steel mesh or rebar would be overkill. It is also used in some old fashioned residential plastering methods for the same reasons. 

Fiber reinforced plaster is an ancient form of what are now referred to as composite materials and the physics are the same.

Alyth Yard

Canada

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Posted by Overmod on Tuesday, October 19, 2021 2:47 PM

Lastspikemike
Often it can help reduce cracking if you introduce fiber into the plaster.

This is good but I'd recommend using a skim coat in the mold first and then introducing the fiber-loaded material behind it.  This was SOP (in reverse) for high-quality wall plaster in the old horsehair days, and now for AVG-style shotcrete fabrication of inside walls without furring and rocking...

I confess to having made regular use of acrylic bonding agents in plaster casting for architectural purposes; it allows for thinner slip and stronger intricate details while also bonding nicely to the 'composite' material away from the face.

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Posted by BATMAN on Tuesday, October 19, 2021 3:49 PM

Brent

It's not the age honey, it's the mileage.

https://www.youtube.com/user/BATTRAIN1/videos 


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Posted by maxman on Tuesday, October 19, 2021 7:28 PM

selector
Also, consider light jiggling, even lifting and dropping short distances

I had one experience to do some plaster moulds when I belonged to a club.  I did the jiggling and dropping thing and can attest to the fact that the air came out.

Unfortunately that did nothing to help the plaster.

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Posted by trainnut1250 on Wednesday, October 20, 2021 11:30 AM

Macman44

 My box of plaster still seems fresh enough, so no immediate concerns there.

I'll also give Lightweight Hydrocal a try when I next buy.

Paul.

 

 

Paul,

(From someone who has mixed 100's of batches of plaster).

The symptoms you describe are from old plaster. It should "go off" (set up) with a definitive heat build up and should become pretty hard. Soft, crumbly castings generally mean the plaster is bad.  Plaster of Paris is softer than hydrocal but still sets pretty hard. If the box has been around for awhile it likely has absorbed water and gone bad.

Experiment with some measuring containers to get a good ratio of water to plaster and then use the same ratio everytime. It makes the process much easier. I mix my plaster to the "soft yogurt" thickness for rock castings.

I dont reccomend WS Light Hydrocal. It is very expensive and I dont like the finished product compared to "regular hydrocal" you buy at the plaster and lath supply store.

 

Have fun,

 

Guy

see stuff at: the Willoughby Line Site

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