Asphalt

Making  surfaces for roads and car parks is something about which there are many theories. Here's a technique that works for me.

Preparation

 * Prepare the area with Gesso (black acrylic primer) and leave to dry at least 24h. I undercoat all baseboards and scenery with Gesso anyway.
 * Mark out the road.
 * Mask the edges with low-tack masking tape, leaving the road surface area clear.
 * Decide whether you need camber, whether the road needs to be proud of the baseboard or not.

If you need camber or if the road needs to be above baseboard level
There are several ways of achieving the desired result, but all of them amount to much the same thing: lay some kind of former, pour in some kind of filler, let it dry, then add the surface. Here's what's worked for me so far.
 * Lay down the road edges with Woodland Scenics paving tape or similar foam tape (e.g. Tesa). Stick this to the masking tape or you may never get it off the baseboard.
 * If necessary use a second layer to raise the edge further - this will allow for a steeper camber (and is also handy for concrete plinths).
 * Double-check that you have everything ready and to hand. The tools you'll need are wet water, spatula, probably a soft paintbrush.  It's very frustrating losing a pot of plaster because you can't find the spatula.
 * Paint the road base with PVA.
 * Wet the road area lightly with wet water.
 * Mix the road base. I have tried plaster of paris, Woodland Scenics' self-levelling road plaster and a kind of DIY version made up from PVA and asphalt scatter.  All work well enough.  I guess the Woodland Scenics stuff is easiest to use, it flows well and does not set too quickly.  If you use plaster of paris consider adding vinegar as a retarding agent.
 * Pour into the road area and smooth level with the spatula. When very wet you can tamp it like concrete using the edge of the spatula.  Wet the spatula before use.
 * As the medium begins to set you can finish it using a wet soft paintbrush (1/2" or 1"). If you use house plaster you can polish the surface using a wet spatula or filling knife.
 * Leave to set.
 * Remove the paving tape (by lifting the masking tape that it is stuck to).
 * Sand to the required contour. Remember that sandpaper will take away vast amounts of plaster very fast, and also leave a huge mess.  I use a dust extractor to suck the dust away.  Sometimes it's helpful to shape a piece of wood to the required contour using a coping saw or spokeshave and attach the sandpaper to that.
 * Remove excess dust.
 * Repair any uneven areas using filler and leave to set.
 * Paint the surface with Gesso

Laying the surface
Start by assembling the tools:
 * A wallpaper seam roller (plastic, not wood).
 * A filling tool or spatula, flexible, flat and preferably stainless steel.
 * Wet water.
 * PVA.
 * Dilute PVA.
 * Paintbrush for PVA.
 * Soft 1" or 2" paintbrush.
 * Pipette.
 * Masking tape.
 * Asphalt scatter.
 * Ink wash with asphalt or black paint for final finish (don't mix until you need it).

Mask the road edges with masking tape. Coat the road surface with PVA (you will probably need to dilute this very slightly to get the glue to spread evenly). You need a reasonably thick coating but not like laying it on with a trowel.

Shake the asphalt scatter thickly over the road surface and roll it firmly with the wallpaper roller. This should result in a firm, even surface. If any glue becomes attached to the roller then the layer of scatter is not quite thick enough, add some more and clean and dry the roller before continuing.

Leave for the PVA to dry, usually 24h.

Using a small soft house painting brush, dress the surface so it is even, smooth and any surplus scatter is removed. There may be small holes that form where glue spread was uneven, you can choose to leave these as potholes (and fill with epoxy "water" puddles) or spread the scatter back over them.

Mist the surface with wet water. You'll know from other uses of scatter that anything other than a fine mist will disturb the surface. When wet, carefully drop dilute PVA over the surface to stabilise. You can polish and perfect the surface by carefully smoothing it with a wet spatula. You cna also form imperfections and potholes, or you can do these with a craft knife after it's dried.

Step back! Don't be tempted to overdo it. If necessary let it dry and have another go tomorrow.

When the surface is dry and done, make up your ink wash and run it over the surface.