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Plaster Disaster

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Plaster Disaster
Posted by Pruitt on Monday, March 29, 2021 9:50 PM

A few days ago I applied some plaster of paris for some scenery approaching the Hudson station on my layout. It didn't really work out. I posted about it in my layout construction thread, and someone suggested I post a separate thread to maybe get some more feedback.

Here's the story:

I filled in the benchwork grid with 1/2" extruded styrofoam sheet, covered it with masking tape and created a very low rise along the backdrop behind Hudson itself. I painted all of that a tan color using the cheapest flat latex paint I could find.

Then I mixed up a thin batch of paster of paris tinted with brown tempera paint and brushed that over most of the painted area:

So far, so good.

Fast forward to the next afternoon...

That thin, brushed-on coat of plaster of paris I applied to the painted substrate didn't stick to the paint at all! It just crumbled off! I expected it to adhere to the latex paint, but it just lifts away, braking into flakes as it comes up. 

I'm really surprised the plaster didn't stick to the paint. They're both water based. The cracking doesn't surprise me, because it's a very thin layer. But I did expect it to stick. Instead, the vacuum just lifts it right off.

Interestingly, the plaster discolored the paint, and also left a whitish residue on much of it.

Anyone have any idea what's going on here? What did I do wrong?

Should I have wetted the paint before plastering? I'm pretty sure the plaster layer should have been a bit thicker, but that probably wouldn't have made it stick any better, would it?

I've put together a test board using a couple different types of plaster on paint, sanded paint, unpainted masking tape, and bare styrofoam. I'll check it out once the plasters are fully cured.

Meanwhile, if you have any thoughts at all, please post them. I'm stumped.

HELP!

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Posted by basementdweller on Monday, March 29, 2021 10:03 PM

Most likely a combination of factors:

Plaster made too wet / sloppy / runny.

Plaster applied too thin.

Typically plaster sticks to pink foam just fine. To be honest I do not like working with Plaster of Paris, I prefer using USG Structo Lite which takes 24 hours to dry, I also like USG Red Diamond plaster for a faster dryer and stronger plaster. 
I use these a lot in my painting business for old plaster repair work so I always have some around. You should be able to find it at a good drywall suppler. 

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Posted by RR_Mel on Monday, March 29, 2021 10:52 PM

Plaster of Paris is very brittle when mixed thin, runny.  It doesn’t have much strength when used under ½”.  I keep my Plaster of Paris thick to prevent cracking.  If you want a thin coat you could go with Sculptamold or Paper Mache.  Paper Mache shrinks quite a bit so figure the shrinkage in to your use.  A 18” section of Paper Mache will shrink over ½” as it dries.  It will warp ⅜” plywood.
 

Mel



 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.

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Posted by wp8thsub on Monday, March 29, 2021 11:43 PM

Pruitt
What did I do wrong?

The substrate you're using is not really compatible with a plaster coat.  Thin plaster needs the be applied over something porous (or otherwise with something the plaster can grab onto) to prevent it from coming loose as it sets.  I've seen other hobbyists try to get plaster to stick over tape like that with similar results.

If you're going to continue using foam board as a base, try switching to a material deisgned to stick to it rather than masking tape.  I've had success using a layer of fiberglass mesh drywall tape, or fabric like bridal veil mesh, which gives something to hold the plaster.  Brush a coat of foam safe contact cement to force the mesh to stick, being careful to leave plenty of voids for the plaster.  As a bonus, properly secured mesh won't come loose over time like masking tape (the adhesive often dries up and fails so your scenery would potentially have been ruined by that eventually even if the plaster stuck).

Bridge Plaster 3

by wp8thsub, on Flickr

Here's a scene where I used fiberglass drywall tape over foam core, applying plaster after the tape was glued down.  Note that anywhere other than the flat river bottom was assembled with fiberglass tape over a cardboard web.

Rob Spangler

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Posted by Lastspikemike on Tuesday, March 30, 2021 8:18 AM

Latex paint may be "water" based but it is an emulsion of "rubber". I very much doubt much will adhere to a latex painted surface that has cured by drying

That's  one reason latex paint surfaces are washable after they cure. Essentially, they seal any porous surface with a thin sheet of rubber. 

Alyth Yard

Canada

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Posted by SeeYou190 on Tuesday, March 30, 2021 9:35 AM

Pruitt
I'm really surprised the plaster didn't stick to the paint. They're both water based.

This is why I prefer the term "water soluable" over "water based" when we discuss things like this.

Water does not evaporate, dry, or remain in the plaster as it cures. It is part of a chemical reaction, and hardened plaster is not the same as powdered plaster/water.

With paint, some of the water evaporates, but some remains in the paint as it cures. Some water soluable paints contain no water. That is why it is always best to thin water soluable paints with the manufacturer's thinner rather than water. Thinning with water can change some properties of the paint.

I would imagine that introducing water soluable paint with the wet plaster could alter the chemical reaction of the plaster hardening process and produce unintended and unpredictable results.

The best solution would most likely be to tint plaster with products intended for that purpose.

Another big concern is the number of people that assume "water soluable" also means "non-toxic".

-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, March 30, 2021 11:57 AM

People are always painting before plastering, bad idea. Sometimes it sticks but many times not as it dose paint, even flat paint dose not have the tooth needed. Plaster is brittle and needs the proper substrata like plaster cloth. Second a lot of people use blue or pink foam, no grab there, I use beaded, lots of holds for grab.

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Posted by selector on Tuesday, March 30, 2021 3:13 PM

I'm thinking that the tape was a mistake, in hindsight of course, and that the paint was as well.  I would have tinted the plaster at the outset with masonry dye or with craft paints (I think the dye works out cheaper), and then painted what needed to be painted after that.

I have to admit that I am surprised as well, Mark.  You'd think it would have set up and bonded firmly to the paint as the paint, itself, dried and set up.  Oh well...

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Posted by hon30critter on Tuesday, March 30, 2021 3:23 PM

Thanks for starting a separate thread Mark,

I question if the latex painted surface was the problem. Plaster of Paris has been used millions of times to patch holes in walls that have been painted with latex paint. It seems to stick fine.

The plaster layer appears to have lifted on its own. That implies that something was pulling it loose. I suspect that mixing the PoP too thin was the main cause of the problem. Adding the Tempura paint may have exacerbated it. If the paint shrank at all as it dried it would have pulled at the bond between the PoP and the surface below, and if the PoP was too thin, its bonding properties might have been compromised to begin with. 

You might want to do a couple of experiments to eliminate some possible causes. Mix up a little bit of PoP in various consistencies and see how well they stick to the latex paint. If none of them stick then the latex paint is the problem. If the thicker ones stick and the thinner ones don't, then the mix was too thin. Then do the same thing with the Tempura paint added to the various consistencies. I'm betting that the thicker consistencies will stick.

I'm not trying to make work for you, but a little testing might answer the question.

Cheers!!

Dave

I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!

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Posted by Lastspikemike on Tuesday, March 30, 2021 6:25 PM

Before testing, lift a piece of PoP off the paint and turn it over. If the paint isn't stuck to the PoP then there's your problem.  If it pulled almost all the paint off with the PoP then the paint bond failed. 

Second idea: PoP isn't very strong. That's why old fashioned broken bone casts used fiber reinforced plaster. That's why the scenery suppliers make plaster cloth. That's also why other types of casting plaster is either spread over a matrix like window screen or contains fibres in the mix.

Mud and wattle, lath and plaster and resin impregnated fibreglass type composite materials all rely on the added strength the matrix material provides. 

For your application you may need to use one of those mixes that already contain the fibre. This chart helps describe the alternatives:

https://woodlandscenics.woodlandscenics.com/show/category/Plasters

 

and I think sculptamold is a paper fibre reinforced plaster mix:

 

https://www.sceneryexpress.com/SCULPTAMOLD-4lb-ECO-PAK/productinfo/EX0093/

 

 

 

Alyth Yard

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Posted by York1 on Tuesday, March 30, 2021 7:43 PM

As others have mentioned, you might try Sculptamold instead of PoP.  I have put on very thin layers of Sculptamold over extruded foam and had no issues.

Let us know what you try and how it works out.

York1 John       

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Posted by Pruitt on Tuesday, March 30, 2021 11:19 PM

Lots of feedback from everyone - thanks!

I'll take a few days to digest it all. Meanwhile...

(sorry if this is a bit long)

As I mentioned, I put together a test board a couple days ago. Here's some detail.

There are four vertical (in the photo) stripes of plaster. From left to right - premixed patching plaster, plaster of paris, another stripe of the same premixed patching plaster, and finally another stripe of plaster of paris.

The left two stripes are on painted masking tape on the bottom half, and painted styrofoam (no tape) on the top half. Same paint as on the layout. I lightly sanded the center of the painted area vertically, so that half the patching plaster stripe and half the plaster-of-paris stripe are on sanded paint.

The right two stripes are on unpainted masking tape on the bottom half, and directly on the unpainted styrofoam on the top half. 

================

I gave the plaster two days to dry thoroughly, and today I took a narrow putty knife to it to see what was what.

The plaster of paris didn't stick to anything at all except the bare nekkid styrofoam. Sanded or unsanded paint made no difference at all - it didn't stick. It also didn't stick to bare masking tape.

Patching plaster shrunk a lot more than the plaster of paris as it dried, but it stuck to everything.

I talked to a local friend (Kurt) about my problems (no, not those ones! The plaster one). He uses plaster of paris all the time. He said he had a similar problem to mine once when he was near the bottom of the bag of plaster. He bought a new bag and had no more problem. His speculation is that the plaster settles and separates a bit sometimes and what's in the bottom of the bag doesn't always set up right. He suggested buying a new container and trying again (I haven't done that yet).

Here he is when he was showing me how he does scenery on my layout about six weeks ago.

The white in front of him is a dried, thin layer of plaster of paris (out of my container) that he painted on the cork sheet that underlays Casper yard. The wet plaster was about as thick as pancake batter. It practically fused itself to the cork. He's painting it with tempera paint.

[EDIT (added 3/31, 0921MT)]: The rest of the cork in the photo I painted with thin, tinted plaster of paris a week or so later and it also worked just fine. [/EDIT]

So,

I picked up a small can of Durham's Watter Putty today. Maybe I'll try a test panel with that (without the tape or paint).

All I can get locally is patching plaster, the Durham's, plaster of paris and celluclay (I used Celluclay on the narrow shelf above Casper. It worked great, but I've heard it's not so good for larger areas).

Hydrocal I have to buy online, and Sculptamold is the same.

My test panel was green styrofoam, because I had a panel of the right size. I think I'll test the Durham's and some fresh plaster of paris on a smaller chunk of bare pink styrofoam and see what happens.

I'm really over complicating this, aren't I?

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Posted by hon30critter on Wednesday, March 31, 2021 12:16 AM

Pruitt
I'm really over complicating this, aren't I?

Hi Mark,

I don't think so. You need to solve the problem so there is nothing wrong with experimenting along the way.

Dave

I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!

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Posted by dknelson on Wednesday, March 31, 2021 9:20 AM

Pruitt
He said he had a similar problem to mine once when he was near the bottom of the bag of plaster. He bought a new bag and had no more problem. His speculation is that the plaster settles and separates a bit sometimes and what's in the bottom of the bag doesn't always set up right. He suggested buying a new container and trying again (I haven't done that yet).

This might well be a big part of the problem.  I have had this issue with other plasters including hydrocal, and including where I thought I had packaged up an open container rather completely.  It may be that even trace amounts of moisture in the air start the curing process with "raw" plaster and make it less strong.  I recall some people reporting that their opened packages of plaster started to emit an almost vinegar-like odor, and that that was a sign that the supply had to be discarded.  

Dave Nelson

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Posted by Overmod on Wednesday, March 31, 2021 10:44 AM

Try this:

(1) use a tinted primer (like one of the Kilz products) as the sealing undercoat;

(2) use acrylic bonding additive in the plaster when you mix it.

There are two separate issues overlapping here: the skim-coat of plaster isn't adhering to the paint 'membrane', and the skim coat of plaster breaks up easily.  These have very different ranges of solution, but both need to be addressed.

It is possible that some of the reinforcement methods used for shotcrete/Gunite (like chopped short lengths of thin fiber or wire) might provide reinforcement for a plaster (or mortar, or self-leveling underlayment, etc.) which may be easier than working with, say, the mesh tape used for spackling.

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Posted by selector on Wednesday, March 31, 2021 10:50 AM

I made a few rock faces using W/S moulds three years ago.  The first set, I used 10-years-open Hydrocal.  I'm not kidding....ten years ago I opened the carton, and I didn't reseal it...just pinched it closed.  The rocks came out prisitinely.

When that ran out, I opened a brand new (at least recently purchased, don't really know if it was truly 'new') carton and had all sorts of issues...bubbles in the castings making them frangible and unusable. I used the same process and materials, too, no changes.

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Posted by Overmod on Wednesday, March 31, 2021 11:05 AM

dknelson
I recall some people reporting that their opened packages of plaster started to emit an almost vinegar-like odor...

Sure indication that a crosslinking polymer additive was provided in the dry product.  An example might be tin-catalyzed alkyl triacetoxysilanes, which are the reason for the 'vinegar' smell when 'acetoxy' silicone caulk cures -- hence the name.

See this reference for other examples of moisture-activated-crosslinking polymer additive systems:

https://pure.tue.nl/ws/files/2130017/200113061.pdf

Yes, by the time you smell the vinegar the additive will have 'reacted' and would no longer contribute positively to bonding or strength...

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Posted by riogrande5761 on Wednesday, March 31, 2021 11:13 AM

I had a plaster disaster on my last layout, but of a different kind.  I tried buttering plaster over plaster gauze and then tried painting it a base color, but the paint pealed off the plaster.  Turns out the plaster gauze sucked the moister out of the plaster it dried very dry.  The latex paint started peeling off.  I wetted the plaster gauze on subsquent efforts and then wetted the plaster after applying.  It seemed to fix the issue.

Rio Grande.  The Action Road  - Focus 1977-1983

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Posted by ROBERT PETRICK on Wednesday, March 31, 2021 11:22 AM

Hey Mark-

I always hate it when someone gives me advice along the lines of " . . . well there's your problem, here's what you should have done." I apologize in advance if this reply sounds anything like that.

How thin of a thin coat are we talking about? Just a skim coat to simulate uneven natural ground surface? Possibly hiding joints in the substrate?

Here's what I would have done. Mix equal parts of drywall mud and el cheapo latex paint of some sort of brownish hue. I'd use white primer if I had some on hand (more bonding agents in primer than in 'regular' wall paint) and add in some brown tinted stuff. The 50-50 mix would have the consistency of thin peanut butter, but it could be slathered on pretty thick. One application should do the trick, but you could add more layers if necessary. This is the same technique house painters use to create crow's foot ceilings.

Robert

LINK to SNSR Blog


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Posted by dew3896 on Thursday, April 1, 2021 6:02 PM

Thanks for starting this thread and sharing your experience.  I'm about to start scenery and your experince is very helpful.  Also, thanks to all who have shared their experince/ideas.

DEW

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Posted by bagal on Friday, April 2, 2021 1:57 AM

I'm kinda baffled by the OP's methods. Foam, masking tape, paint then tinted plaster. What's the point of the masking tape?And why paint before plaster?

I'm foam, skim coat of plaster as necessary, paint then ground cover. I'm not even sure there is any need for paint.

 

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Posted by Pruitt on Friday, April 2, 2021 10:31 AM

bagal
I'm kinda baffled by the OP's methods. Foam, masking tape, paint then tinted plaster. What's the point of the masking tape?And why paint before plaster?

The masking tape was to cover gaps between the styrofoam and the backdrop, subroadbed, and mating styrofoam pieces. I simply continued to cover the rest of the styrofoam as well, in keeping with the idea of Joe Fugate's use of a continuous masking tape layer (in lieu of plaster cloth) over cardboard lattice. Same for the painting of the masking tape. This was all presented in a series of DVDs he produced 12 or so years about about construction of his model railroad. He added cement and vermiculite to his plaster mix, which may be why he didn't have the same problems I did.

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Posted by Pruitt on Friday, April 2, 2021 3:03 PM

On the 31st I made the test panel with Durham's, and with a new container of plaster of paris and the old one (just to see if maybe the old batch was bad).

Last night I checked to see how well they stuck to the styrofoam. Not well, as it turned out:

A putty knife slide pretty easily between the styrofoam and all the plasters, breaking the plaster away with no damage to the surface (except where I gounged with the knife a little).

So I roughed up the surface and made another test panel:

I'll see this evening how this one works.

This is really disheartening. Seems that people are building scenery by putting plaster on pink foam all the time. Why am I having so much trouble?

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Posted by KitbashOn30 on Friday, April 2, 2021 9:04 PM

Pruitt
This is really disheartening. Seems that people are building scenery by putting plaster on pink foam all the time. Why am I having so much trouble?

Aw man, that's a huge bummer. Sad

Sorry I've not got a solution to give, right now the best I can offer is a question,
if Plaster Disaster were a band name, what genre would they play?

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Posted by wp8thsub on Friday, April 2, 2021 10:56 PM

Pruitt
This is really disheartening. Seems that people are building scenery by putting plaster on pink foam all the time. Why am I having so much trouble?

For what it's worth, I've seen a lot of bad scenery built over foam where the plaster bond has failed.  I haven't had great experiences with it either, hence my preference for other methods.

Rob Spangler

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Posted by ATLANTIC CENTRAL on Friday, April 2, 2021 11:02 PM

I wish I could offer some help, but I have zero experiance with foam as a base for scenery or track.

Like Rob,, I still prefer older/other methods. 

Sheldon

    

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Posted by Doughless on Saturday, April 3, 2021 11:09 AM

There must be other products that have plaster of paris qualities but stick to foam.

Extreme fix would be to use foam friendly construction adhesive to attach thin veneer, cardboard, or cardstock to the foam, then plaster of paris would likely stick to those more fiberous sheeths.  Would be a PITA though.

- Douglas

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Posted by selector on Saturday, April 3, 2021 7:18 PM

Mark, I made a 'paved' street using yellow glue mixed with some craft paint and PoP.  It stuck rather well.

I grooved the surface of the extruded foam first, then painted on a layer of the goop, as if it were good quality wall paint. I know it's a coarse scene, but the 'pavement' was fairly durable as long as I didn't poke it with something sharp.  Your application should be fairly strong, but the yellow glue did the trick for me.

 

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Posted by Pruitt on Saturday, April 3, 2021 8:13 PM

Thanks for all the support, everyone. I really appreciate it.

Selector, I checked my roughed up foam test panel this morning, and the plaster of paris really grabbed onto that hard. Like your street, I think that's my solution. I still need to cover the gaps, but I think I can do that using masking tape just on those seams. I'll just make sure the plaster isn't real thin in those areas.

Thanks again for the suggestions and good thoughts, everyone! Thumbs Up

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Posted by bagal on Monday, April 5, 2021 5:18 AM

The tape and paint over foam doesn't seem necessary to me. I have simply applied patching filler over the foam to smooth the landform and fill gaps, then paint.

 

 

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