To spray wood glue or matte medium, you need to dilute it down to a 20% (1 part glue/matte medium - 4 parts alchohol) or even down to 10%) as anything less will plug up the sprayer. Add a couple of drops of soap to the mixture.
Purpose of Soap
Glue or matte medium, by itself, regardless of how well it is diluted, will blob on the ballast or ground foam scenery and will not soak into the dry ballast or scenery. There is "surface tension" between the ballast/scenery and the glue/alchohol/water. A couple of drops of soap in the mixture will break that surface tension.
As an experiment, you can float a paper clip in a glass of water and the paper clip won't sink to the bottom. Add a small touch of soap and the paper clip immediately sink to the bottom of the glass. The soap breaks the surface tension between the paper clip and the water so that the paper clip plunges to the bottom of the glass. Same thing with the ballast/scenery, your base scenery (plaster/Styrofoam/etc), and the glue.
Why Use Alchohol?
Alcohol will evaporate faster than water. It is available at your local hardware / paint / building supply store at a very reasonable price. I've applied one layer of ballast/ground foam scenery in one evening and by the next evening, the ballast/scenery is rock hard, ready to apply the next layer of scenery.
Painting The Scene
Depending on the method you have used to create your scenery base, you will be staring at white plaster, pink or blue styrofoam, or some other basic colour. Some scenery books use the features of the plaster as a start for scenicking. Because I'm into module railroading and I use styrofoam and spackling compound to create my scenery, I don't have that luxury as I'm staring at a whole bunch of pink and white. As a first step, I take a flat grey or brown latex paint (the quart size available for about $15 at your local paint store) and apply 2/3 coats to the base scenery to get rid of that white/pink/blue/etc look. The colour you use will depend upon the type of scenery you will be creating - grey sedimentary rock, brownish igneous, prairie, etc, etc. Once I've applied these coats, I let it all dry.
Drybrushing Latex Paint
Another trick is to dry brush some "accents" into your scenery. As I use styrofoam cut to shape with a hot wire and painted with grey paint, I end up with nice "ridges" in the scenery base. I can accent these ridges by lightly dry-brushing a darker grey over these ridges. I simply dip a 1" brush into the paint, brush out the excess onto a piece of newspaper, and then drybrush the painted styrofoam ridges. The operative word here is "dry". While a lot of this will be covered with scenery, there will be enough that will be left bare, particularly on the steep slopes.
While the paint is still wet from dry-brushing, I apply some earth-tone scenery (see below for techniques). Some of the ground foam will stick to the dry-brushed paint while some of it will fall to the bottom of the slope. The effect is quite outstanding!
Applying Paint Effects
Painting the scenery with two coats as described above will produce a uniform monotonous look to the scenery, the objective being to cover up the pink and white. Here's where applying a little bit of mixed paint will produce some nice effects. My base coat is light grey representing sedimentary rocks. I lightly dab a 1" paint brush into some flat latex brown and mix it up on a piece of paper or index card. I then lightly touch the paint brush into some flat latex black and swirl it into the brown. If I have too much paint on the brush, I simply brush it out on a piece of newspaper. I then lightly brush this mixture on the end of the paint brush onto the scenery creating "accents". Where you create the accents will depend on what the slope and contours of your base scenery will look like. While the paint is still wet, I apply my first layer of ground foam scenery.
I do the "dry-brushing" and the "paint effects" both at the same time, covering no more than a 2'x 2' or 3'x 3' area in one session at a time. This includes applying the ground foam scenery.
I apply the scenery first and the ballast last. If I apply the ballast first, followed by the scenery, I find that the scenery will end up on the ballast. While this effect might be okay for an unused siding, it's not the effect that I want for my busy railroad.
Take Your Time - Savour the Experience!
Don't try to scenic your whole layout at one sitting. You're going to get tired and you are going to lose those "creative juices" on where and what colours you want to apply, particularly in applying those subsequent layers of ground foam scenery. Take your time and enjoy the process of creating your scenery. You may even want to go out and buy one of those artist's berets and white smocks to get you in the mood (VBG!).
Doing scenery is a creative process. Be well rested and in the mood to create the scenery. If you aren't in the mood, don't do the scenicking that evening. Do something else. Save the scenicking for another day when you are in the mood. You probably don't know what I'm talking about and you probably think I'm off my rocker when I talk about scenicking being a "creative process". But you will when you start applying the scenery. You have to go through the experience to understand what the creative process is all about.
Don't try to scenic or ballast your layout in one evening. It won't work. Do all of your scenicking first and then apply the ballast. Or, scenic a large area of your layout and, after all of the scenery has been applied to that area, then apply the ballast. I usually scenic a 2'x 2' or a 2'x 3' section of my modules in an evening. Or, I apply one layer of ground foam in one or two evening before I apply the next layer of ground foam.
Scenery Research
I'm not an artist. Once I get beyond the basic colours of Red, Orange, Yellow Green, Blue, Indigo, and Violet, I'm completely "colour blind". This is where some "scenery research" comes into play. Before starting all of this, I take photos of rocks faces, slopes, ground surface, etc. This helps with the creative process in applying the scenery.
Even then, I'm still colour challenged. So, I call on a colour expert - my wife - to mix up some colours for me and to recommend which colours I should apply first. A good way to get your spouse involved in this hobby - even if it's only for a few minutes of consultation and mixing.
Applying the Ballast/Scenery
Ballast/scenery is applied dry. I mix up the ballast scenery in paper coffee cups as I can put a plastic lid on the cup when I've finished applying the ballast/scenery. I use a 1" paint brush to move the ballast/scenery around. Hold the coffee cup in one hand and tap the back of your hand with the wooden handle of the paint brush. This way, you can better control the application of the ballast/scenery to the roadbed/scenery base.
For example, tap some ballast into the middle of the tracks using the above technique - not too much. It's easier to apply more ballast than to remove the ballast from the tracks. You will end up with a ridge of ballast in the middle of the tracks. Use the brush to move the ballast in place. No need to drop ballast to the outside of the tracks as the brushing will automatically pour over the sides of the tracks and onto the outside. On steep slopes, lay down a thin strip of glue (50/50 or 40/60). This will catch the loose ballast and hold it in place.
On ground foam scenery, use a light touch with the brush. It's best to apply the ground foam scenery in layers as you can look at the finished product before deciding the next colours you want to apply.
Another trick is to "blow" the scenery into place. I use a "turkey baster" that has the rubber bulb on the end. A very light squeeze on the bulb (the operative word here is "very light") will blow the blobs of dry scenery into place, off of the tracks, or out of where you don't want the scenery to be.
Wet The Ballast/Scenery Well
Once you have your dry ballast/scenery in place, soak the ballast/scenery with alcohol. I use a spray bottle with an adjustable nozzle. The mist setting works the best. The alcohol should almost be flowing out of the ballast/scenery. Spray upwards and let the mist float down. Applying the alchohol directly will blow the ballast/scenery out of position.
Apply The Glue/Matte Medium
There are various ways to apply the glue/matte medium. Regardless of which way you go, the glue/matte medium needs to be diluted. I've used squirt bottles and I've also used spray bottles. My preference is spray bottles but that's my preference and may not be yours. A lot of glue/matte medium isn't required as it will be distributed throughout the wetted ballast/scenery applied in the previous step. And, if you apply your scenery in layers, you will be applying more glue to that which has already been laid down.
When you have finished spraying the ballast/scenery, it will look as if you have had a scale 10" of rain fall on your layout. Have no fear! It will have all disappeared within 24 hours - which is the main reason I prefer to use alcohol instead of water.
If you use a dropper or squirt bottle to apply your glue/matte medium, it may "blob" into little white puddles, even though it is absorbed into the ballast or scenery. Simply spray the blobs with alcohol until the blob is diluted into the scenery.
Keep The Glue Out of the Switch Points!!
Be careful how you apply the glue/ matte medium around the switch points. Regardless of how much care you take, the switch points will probably glue up due to the "wicking action" of the glue/soap/alcohol/water. To reduce the amount of glue, take an index card, fold it in half, cut it to size, and place it over the switch points.
If the points are glued, don't force them! Squirt some alcohol on the points, let the alcohol stand for 5 minutes, then move the points. Wipe the glue off of the contact points. This may take several operations.
Applying Subsequent Layers of Scenery
I like to apply my scenery in different layers, starting with the earthy colours. I glue the earthy colours in place using the above procedures, letting the earthy colours dry. I can then stand back and try to visualize what the ground should look like next. I then apply the darker fine grassy colours, working my way up to the coarser and lighter colours.
Earthy colours will cover the complete layout whereas the coarser lighter colours may cover only parts of the layout or the scene.
Clean the Rails!
The glue/matte medium will definitely stick to the rails and will insulate the tracks. Cleaning the tracks is a multi-step process. I start off with a J-cloth soaked in alchohol and wrapped in my index finger. I rub the J-cloth along each rail. After this, I use my "Bright Boy" to polish the top of the rails. Close examination will clearly indicate where blobs of glue may still be clinging to the rails. Be careful around the switch points as some turnouts can be very fragile.
Remember, the idea in applying scenery and ballast is to have fun. The scenery and ballast, combined with weathering the rails can produce a most dramatic effect to your layout.
Bob M.