My thoughts

You could always try and use things like dental picks etc to carve a little of the unrealistic appearance out first. Harbor Freight has them cheap or get some free from your dentist- they go through a ton of them.  Try a hidden area or cast a practices set first to be sure you get a look closer to what you want.  Worse case scenario is you have have to make do over which isn't too bad - we all have those anyway.

For painting cast rocks with paint I like using flat craft paints (cheap from Walmart etc) and thin them very well. Start with a 1:1 wet water to paint. Depending on the look you are I would get white and black to make your own gray or you can buy a shade you like.  You will need a light tan too.  You could add a touch of mustard color or sienna as  highlights.   Get a picture of close to what you are thinking to keep you honest as you go.

I like to apply the paints with with old spay bottles (like 409 bottles) 1 with gray and the other with the tan.  Spay the gray on and let it flow into all the cracks.  A t the same time spray the tan (less tan than gray).  You can always have a 3rd bottle of black mix to use to dark if it gets too light but I would be careful with this.  This gives a very realistic look to the rocks and no 2 areas look alike.  I would try this on scrap or a practice setup area first.  It will lighten a little as you go.  It is easier to add more in a second round as you can not remove too much. I would NOT use a base coat of white. 

It is a messy process so protect anything around the area.  It will pool in low areas and leak to the floor so cover that area too

Steve

A Common Problem

What you've encountered is something I see a lot - the plaster mix used for the castings ends up with a different density than the plaster used to fill between them, so the ability to absorb stain varies.  Despite so many in the hobby apparently thinking Hydrocal is magic fairy dust of some kind, any setting-type plaster will work for rocks, and any can also have the problem you describe.  Don't worry about the brand name of plaster you use.

I carve all my rocks (from casting plaster), so don't have the variation from cast vs not, but perhaps my basic painting method will help.  I start with a wash of black acrylic, followed by layers of dry-brushed undiluted acrylic building from dark to light to impart some depth to the color.  I'm not depending on the plaster being able to accept a stain.  I described the process on my blog here https://forum.mrhmag.com/post/doin-the-desert-scenery-and-backdrops-on-the-8th-sub-12189811. 

To combat what you're experiencing with the varying absorption, you could try a more opaque coat of black or another dark color before starting the lighter stuff.  I like to get the black pretty dark first, but you don't see much of it when everything's done.  Here's some typical finished rock on my layout:

Rock painting feedback

Hi Rob,

Thank you for the input. I had a look at your blog. Really good stuff! Only problem  is I do not have access to the paints you mention. I am sure I can use the normal acrylic tubes ie burnt umber, etc. The dry brushing could be quite a bit of work considering the cliff area that I have to do is in access of 2 meters in length. See photo below..What you see is the work in progress . I did as you advised and used a stone gray pigment to darken the "white" areas between castings. This seemed to work ok.Unfortunately my rocks ended up a bit dark.

This is my first attempt using the pigments. Any suggestions as to what would make the "work in progress" look like the real deal or should I start again?The layout is basically a coal line servicing a branch line, so the rock color could be ok I guess.  

Rocks

Thank you Steve. I am looking at all my options. So many ways of doing this man. Much trial and error. I will do some more experimenting with the acrylics . I would appreciate any other comments on the photos.

Re: Feedback

That's fine.  I use the craft paint since it's cheap and lets me avoid mixing.  Some of the pre-mixed acrylic artist's colors should work just about as well.  A color called "unbleached titanium" is a very useful light tan to add to the more typical colors, plus there are neutral grays and other colors depending on what you're trying for.  I use a dark, medium and light shade of the desired rock color.  Apply in three passes over the initial black/gray, darkest first.  Some people like to finish with white, but I find that too stark in most cases.

Nah - its' FAST.  Probably faster than the washes, as I used to use those before stumbling on my current methods.  Try brushes of various sizes to see what you're comfortable with.  You already have much of the base color that you'll need.

Not to worry, dark right now is GOOD.  I often even go darker to start, but what you have is fine.  The rocks in the photo I included above started off as dark or darker than yours.  You eliminated the overly light mismatched areas, and are ready to start adding color.

Rock painting feedback

Thanks Rob.

Forgive me for all the questions. Its just that I am not a color guy, so I find this whole process a little daunting. Nothing like experience hey.

With the dry brushing, do you cover all of the rock or a little here and there?

Peter

Coverage

I mostly use downward brush strokes, and just let the brush hit what it will.  It should probably cover most of the initial dark color with the first pass, a bit less on the second, and not much on the last.  Cover until you have something that looks like rock and quit.  There isn't really a guideline other than the appearance of the finished product.  You may want to experiment on a separate rock casting first.

I use paint, diluted at least

I use paint, diluted at least 9 to 1 water to paint, dabbed on with a cheap foam brush.  First a few dabs of Burnt Umber, then some overlapping dabs of yellow ochre, then the entire rock face in black.  If it's too light I add another coat of black.

Advice painting rock castings

One more question

Liquid acrylics, diluted into

Liquid acrylics, diluted into a wash.  One cheap Dollar Store one inch foam brush for all colors.  Always just dabbed on.  No brush strokes.  I do not wait for the first colors to dry.  I want them to blend.  The black will run into the cracks and low points, making them naturally darker.  The wash soaks into the plaster.  I use Hydrocal for most rocks.

Painting rocks

Thank you so much for advice.I will give it a bash.

Here’s my 3-step (just about) Foolproof method

Before yesterday, I'd never colored any rock castings & the only ones on the layout were done by others. But modeling a line set in Southern New England pretty much guarantees that I have to figure out how to do rocks at some point. So, now that I'm working in Dividend and will be modeling a railroad cut through a hill, it was high time to figure out how to do it.

Bare castings

I'd made some rock castings some months ago - a pretty easy process, all things considered - but they'd sat for months in the glaring white of new plaster. So, figuring I had a bunch of "extras" to practice on, I did some quick research in my extensive library of scenery books and got to it.

The same castings, now colored

In the process I discovered a technique for coloring rocks that's ridiculously simple and produces rocks that look very similar to what I see along my RR ROW everyday. I've only been coloring rocks for 2 days, so I'm certainly no expert - but I hope by sharing this super-easy technique that you'll be encouraged to try coloring some rocks too. It really is as easy as 1-2-3.

Part 2

.

The same castings once all dry

But first, you'll need some materials.

• 3 Colors of Acrylic Craft Paint
  • I used Raw Sienna, Raw Umber, and Black ($.50 ea at Walmart)
• Water
  • Regular tap water is fine
• Brush
  • I used a 1/4" wide cheap china brush
• Small Dixie Cups
  • Bonus if they're plastic, so you don't have to worry about them getting soggy
• Medicine dosage cup
  • Optional - I have plenty on-hand and the measurement markings are handy
• Stirring stick

Preparation

• Measure out 1 tablespoon of water, and pour it into one of your cups.
  • Put 20 drops of Raw Sienna in the cup and mix it by stirring thoroughly
• Measure out another tablespoon of water, and pour it into your second cup.
  • Put 20 drops of Raw Umber in the cup and mix it by stirring thoroughly.
• Measure out a final tablespoon of water, and pour it into your final cup.
  • Put 7 drops of Black in the cup and mix it by stirring thoroughly.

Here's what you should have when you're done - 3 cups of very thin paint which we'll use to "wash" over the rock casting:

Part 3

Coloring - Easy as 1-2-3

Here's the rock casting I started with - all nice and white plaster, but doesn't look like a rock.

Step 1 - Brush on Raw Sienna

Literally, just dip your brush in the cup of Raw Sienna wash and brush it all over the casting. If you get any pooled wash, just wipe your brush off on a paper towel and use the brush to daub up the puddle. When you're done, the casting will look like this:

Before you go to the next step, you need to let the casting dry. Acrylics dry relatively fast, but if you're impatient like me, you can use your wife's a hairdryer you bought just for such things to blow dry the casting much faster. Here it is once it's all dry:

Step 2 - Brush on Raw Umber

Yup - same "technique" - just dip your brush in the cup of Raw Umber wash and brush it all over the casting, using your brush to daub up any pooling as I described above. Here's the casting right after applying the Raw Umber wash:

And here it is after blow-drying:

Step 3 - Brush on Black

Can you guess what comes next? RIGHT! Just dip your brush in the cup of Black wash and brush it all over the casting, daubing up any pooling. The black really makes things start to pop. Here it is semi-wet:

And - voila! - here it is after blow-drying:

That is really all there is to it! Compare this "rock" to the stark-white plaster casting we started with. There's really no comparison. All of the variations of color really bring out the texture and make it look authentic. And, interestingly, different castings also take the colors a bit differently (as does different types and ages of plaster, apparently).

All that's left to do on this rock to is to weather it (if desired), maybe do some drybrushing to create more highlights, add some "mosses" and other vegetation (ground foam, etc), and - of course - place it on the layout. . . probably somewhere near the front, where such a wonderful bit of modeling can be seen and enjoyed.

You may also want to be sure and seal the castings to keep them from fading over time. I used Woodland Scenics Scenic Cement, but any clear flat acrylic will do. You could also use the sealing medium - if it’s an adhesive - to add the mosses and other vegetation.

I'm almost embarrassed to admit how long I'd put off coloring rocks - you can bet I would've tried it sooner if I'd realized it'd be this easy to do. Of course, this is just one technique - there are certainly others out there and you can play around with different colors to suit the locale you're modeling.

I hope seeing how crazy easy rock coloring is will encourage you to try it for yourself. And if you do, I hope you'll let us know and share your results with us in the comments below.

Happy Modeling & Rock On!*

*sorry - you didn't really think I could resist, did you?

Rockwork Painting

I build a lot of rockwork models for themed landscape architecture projects; I typically hand-carve them from urethane foam, but the painting process I use is the same for plaster castings.

The first thing I do is seal everything with a coat of matte black interior house paint from the hardware store. Use a big brush to spread plenty of paint, then immediately work over the wet paint with fine brushes to make sure you've pushed paint into every crevice - you don't want any white showing.

Once the black is dry, paint over everything in your base colors. You want to try to cover everything pretty well, but don't have to be as through with the crevices; any black left showing looks like dark shadow and adds to the appearance of depth, especially useful with typical interior layout lighting. Blend your base colors as needed, dab colors on, and build up layers of color. Once this is dry, thin down various colors with water into thin washes and spritz them on with spray bottles. Mostly dark brown tones, but also oranges and yellows, and hit a few areas with some light colored (not white) washes as well. Finally, use some thinned paint to add a few details like some veining and you're done. 

It takes some practice, but not much. And if you don't like the results, you can use your base colors to repaint it and try again. Having plenty of reference photos on hand while you paint helps ensure realism.

These are a few pics from a model I built last year:

Rocks

Chris -  Very helpful and well-presented.  Thanks for posting your " how-to". 

Elwood - Excellent work.  Thanks for sharing your technique and photos. 

DEW

More Rocks

wp8thsub - I've always enjoyed your work.  Thanks for posting your rock painting tips.

DEW

Thanks DEW

I appreciate the kinds words.

The scene pictured earlier has changed some and now looks like this.