plaster of paris with WS colors not staining properly

[applor]

Hello everyone,

 

I am a long time lurker but I have never posted - so hello to you all.
I am in Brisbane, Australia and modelling 1954 Germany.

My current problem is regarding painting plaster, a topic that is relevant to a lot of modellers.  I have posted this issue elsewhere but I think the members here probably have more specific experience since I am using Woodland Scenics earth colours.

So to the problem:

I am casting moulds with plaster of paris and finding my WS earth colors are washing over the plaster instead of staining.
My first few attempts stained perfectly and I went full steam ahead, pouring dozens of moulds that I need.

After that first batch I have tried painting with poor results because the paint is running off instead of staining:

The flagstone platform top left was my early test paint and came out excellent.  Compared to the same technique in the bottom right, the paints washed instead of stained.

The rock face in the bottom left also soaked the paint correctly but the other walls and tunnel portal all had varying levels of paint soak in and look over washed while the surface remains somewhat colour free.

Two more examples where the paint has not stained:

In each of these cases I prepare my plaster of paris to a thick cream consistency (I do 2/3 cup water to 1 cup plaster) and the casts have all set perfectly and allowed to cure for a minimum of a week before painting.

I am using Woodland Scenics earth colours, diluted to 1:16 in each case, as per WS instructions and following the 'leopard' spot technique.
I have not used any sealer at all before or during painting.

It was suggested at first that my use of a wetting agent to prepare the moulds might be sealing partially sealing the surface.
The theory fit perfectly so I tried without any wetting agent but found the same result, so it was not the cause.
I also thought maybe it was my plaster to water ratio, so tried a mix as thick as paste (1 cup water to 2 cups plaster) but this made no difference to my painting problem either.
The PoP was all the same brand bought in 2.5kg bags at a time.

Currently the only cause it seems is that the different PoP batches are of differing quality and the cause.  Another user on stummi forums had found that to be the cause when he had this problem.

Has anyone encountered these same difficulties?

I have wasted many hours on this issue now, along with dozens of casts that I can't paint.

Assuming that it is the plaster, given there is no other explanation, I have two options:

1) try a stronger WS paint blend, or mix with matt medium so that it paints on the surfaces instead of washing into the cracks (or both)

2) purchase a different casting compound - a shop nearby sells hydrocal for a reasonable price.  Quite possibly will resolve my problem but theres a chance it won't.  I could buy WS lightweight hydrocal but its just so expensive here ($30AU for a ~900g carton!  I pay $5 for 2.5kg of PoP or hydrocal $24 for 5kg)

 

I am very interested to hear your thoughts and experiences and appreciate you taking the time to read this rather long post.

[wjstix]

Well, what was the old caveat in ads..."your results may vary"? I'd say if the 16:1 wash is too weak, try watering it down less. Even something as basic as the water you use can be different in hardness than other people's water, which can affect the plaster and the colors a bit. Be sure to shake the colors while still in the bottle, preferably in a hobby paint shaker, to make sure you are getting the full strength colors before adding water.

[RR_Mel]

I’ve been staining PP with WS Stains since I first started my scenery back in about 1993.  I kinda remember having similar problems back then.  For the last 10 or so years I’ve been using a 1:8 ratio with the WS Stains and no problems, can’t remember what ratio I used in the early 90s.

 

I do remember having problems with the plaster not absorbing the stain and that it was caused by the brand of the Plaster of Paris.  I remember having problems with the plaster from the craft stores.  I have been buying my Plaster of Paris from big box hardware stores for years and no problems.  I’ve had really good luck with Dental Hydrocal for my light weight castings off eBay, much cheaper than the hobby suppliers and I like it better too.

 

About 10 years ago I went to using cheapo glue brushes to apply and spread the WS Stain, I get my brushes at Harbor Freight.  The stiff bristles of the glue brushes seem to work better for me.   

 

 

Mel

 

 

My Model Railroad   

http://melvineperry.blogspot.com/

 

Bakersfield, California

 

I'm beginning to realize that aging is not for wimps.

 

[rrinker]

 From the photos it looks about like it should, from videos I've seen showing the technique. Usually you need multiple applicaions to get it where you want it - better it be too light than come out too dark with the first application, you can make the lighter darker but making darker lighter is next to impossible.

                                           --Randy

 

[rrebell]

The weather changes and so dose the dry times. Once had a problem with zip texturing and even talked to those who should know about it as they are writers of such things. Turned out I let the plaster dry out too far, same was true of staining I did (one discovery led to the other). Also discovered regular plaster could be stained much later than Hydracal. Do your own experiments, just put blobs of plaster on something and paint at different time frames and see if you get what you want.

[kasskaboose]

You follow my approach-- painting with dilluted cheap craft paint once the PoP dries.  That is a cheaper option.  Perhaps try that approach on a small rock formation and see for yourself.

[selector]

W/S hydrocal tunnel portals have always taken stain well in my experience.  I haven't stained plaster of Paris much, but have you by any chance sprayed the castings with Woodland Scenics Scenic Cement at any time?  That seals the surfaces and will cause much more variance in adhesion and movement of the stains.

[RR_Mel]

selector

W/S hydrocal tunnel portals have always taken stain well in my experience.  I haven't stained plaster of Paris much, but have you by any chance sprayed the castings with Woodland Scenics Scenic Cement at any time?  That seals the surfaces and will cause much more variance in adhesion and movement of the stains.

 

Good point.  I eroded badly early on by sealing my rocks.  I had fluorescent lighting in the garage where my layout spends it’s life and over the years the fluorescent light faded my scenery badly in less than 10 years.  It was somewhat easy to replace the WS flocking but the sealed Plaster of Paris rocks were a goner!  I had to replace all my rock castings because I stupidly sealed them with WS scenic cement.

 

After replacing about 12 square feet of rocks I didn’t seal them.  Ten years later I had to re-flock the WS flocking and it was an easy task to re-stain the rocks because they weren’t sealed.

 

I have replaced the fluorescent lights with LEDs so no more fading, problem solved . . . . I still never seal my Plaster of Paris and Hydrocal rocks just incase.  

 

 

Mel

 

 

My Model Railroad   

http://melvineperry.blogspot.com/

 

Bakersfield, California

 

I'm beginning to realize that aging is not for wimps.

 

 

[applor]

Thanks everyone for taking the time to read my post and respond.

As mentioned in my post, I have never used (or even have) WS scenic cement or any other sealer in my moulds or on my casts.
Also, I always thoroughly shake my WS color jars to ensure all the sediment is dissolved.

A few of you have confirmed it is related to the plaster, so that is good to know.

I am trying to determine the best way forward to minimize lost time and money.
Currently I have 8 large jars of WS colors all diluted to 1:16.  I can try some stronger mixes to see if it improves my painting but I don't want to tip out all my paints.
Also, I have a dozen tunnel wall casts and a dozen platform pieces that will likely have this issue, so I don't want to throw them out and waste all that plaster and casting time either.

My big concern is inconsistency, so if I start painting platform pieces with a strong blend and it looks good but then a piece is more or less absorbant it will look different and out of place.
The answer to that of course is to use a primer but that will prevent me being able to stain.  More so I don't know what primer to use on plaster and if water colours will work at all, since primers are usually designed for latex or acrylic paints.

 

[RR_Mel]

You might try some diluted Muriatic Acid to open the pores on the plaster.  I’ve used Muriatic diluted about 1:10 sprayed from a fine sprayer, very lightly to make the plaster accept stains better.  Test it first on something to make sure it will work on your plaster.

 

Mel

 

 

My Model Railroad   

http://melvineperry.blogspot.com/

 

Bakersfield, California

 

I'm beginning to realize that aging is not for wimps.

 

 

[applor]

Well I've tried painting with a stronger mix of WS on bare plaster, about 1:4 ratio.  Doesn't come out well as the colour is still spotty/splotchy.

Shown below on the bottom right platform.

 

I tried using a primer shown below in the bottom left.  The top part of the platform I used 1:16 stain and its so weak you can't see the colour.

 

The middle section I used the 1:4 stain and it looks OK.  The primer has a brushed texture from applying it which shows up with the stains - not sure if its good or bad.

 

Lastly I decided to try and stain a rockface I casted a few weeks ago.  It was a new mould with no flow agent - it stained perfectly.

I had a platform cast at the same time (same plaster batch, same mix, no flow agent) and since the rockface stained perfectly I thought it may stain too.

Unfortunately as you can see below (the top platform) failed to stain.

 

 

Here is the rock face as comparison.

 

 

This doesn't really make much sense to me.  Perhaps the repeated use of a flow agent on the platform mould is somehow causing this?  Even though I have not used any for a few casts now.

I don't have any other explanation as to why the platform failed and the rockface stained so well.

My last option is to buy a bag of hydrocal and see if it makes any difference on the platform.

Otherwise I have no choice but to use a primer on my platforms or a different type of paint.

[Medina1128]

I don't know if you have Walmarts down under, but they carry a line of acrylics in their craft section made by Apple Barrel. They are thick but watered down, they work great.

You can also find them on eBay or the Dick Blick website carries a wide variety of acrylic paints. Dick Blick art supplies

[applor]

Well I tested the back side of that platform plaster piece with my water colours and they all stained perfectly. Seems my problems have to be the molds themselves. Either silicone leeching into the plaster or the same thing but from the repeated flow agent usage previously. Perhaps cleaning the molds might resolve the problem. Would warm soapy water be the best, or something else?

[rrebell]

Wait, you used something in the molds, not just plaster? If so, that is your problem.

[BigDaddy]

rrebell

Wait, you used something in the molds, not just plaster? If so, that is your problem.

He tried it both ways

applor

so I tried without any wetting agent but found the same result, so it was not the cause.

[rrebell]

Once contaminated, hard to clean off aditives.

[applor]

A few days ago I washed out all my moulds with warm soapy water that had been used with Lukas flow agent.

I then did a test batch and last night they were cured enough for me to paint.  The results:

As you can see cleaning the moulds was successful for the most part.  There are some patches and areas that were still sealed but another more thorough clean will fix that.

It is quite striking the areas that would not stain.  After the success with staining the platform I then painted the other casts where the moulds had been cleaned and experimented with colours:

Base coat for red brick

 

So in conclusion I am very pleased that the cause and solution has been found - but it has cost me months of work.

Thankyou all for your assistance in identifying this problem and by chance if someone is using water colour stains then hopefully they will see this and avoid the same trouble I had.

From now on I will avoid the Lukas wetting agent and use only pure alcohol.

[BigDaddy]

Once you have a link ending in .jpg you can use the picture icon to post the pics directly.  Glad you figured it out.

[applor]

Sorry I have added the IMG tags.

[BigDaddy]

No need to be sorry, you didn't know, know you do and can teach someone else.