Plaster Disaster

[rrebell]

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.

[SeeYou190]

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

[wp8thsub]

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.

[RR_Mel]

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.

[basementdweller]

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. 

[Pruitt]

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!