Step 1-Surface creation
I was looking for an effect that wasn't too ordered and portrayed a visual look that suggested the area of interest was not confined to the squarish wooden base but more than that , you just couldn't see it from where you were looking. Having a square edge to the concrete would give the floor more attention and presence than I wanted to have, so I went for a uneven edge, where the finer details [of the scene] would spill on to the wood, as though they are at the edge of the scenes own reality. Sorry if this sounds deep and a bit stupid but I don't know any other wasy to describe what I wanted to create.
I started by applying a relatively thin coat of multi-purpose filler (water based) and mixed it to a 'porridge like' consistency.
I spread this out flat in even strips with a wide flat putty applicator that you can get at any hardware. Once the area was covered evenly I applied more to certain ares and smoothed it off in different directions until I was happy with a pattern that suggeted squarish patterns like hand laid concrete. When its done manually, long poles (like brooms) are used with flat wide wooden ends to smooth out the bumps, and these leave tracks.
I also encouraged certain areas to have spots 'pull away' from the surface and this was accentuated when nearly dry with sand paper (explained further on). Please see detail thumbs for a closer look.
The edge was shaped when the filler reached a consitency of very soft plastiscene and the exposed wood wiped with a damp rag. The reason why I use this particular filler ( Agnews water putty, made here in Melbourne) is that it is really easy to work with, non-toxic, water based, easy to clean-up and it drys rock hard. The final reason and probably the most important artistically is that it has that very light sand color which allows you to stain it any way you want.
Once the filler is at about 90% dry but still has a small amount to dry along the top, press a very course sheet of sand paper into the top. I used a round weight to roll over the paper while it was face down because it was quite difficult to get an impression- that shows you the consistency you wait for as you don't want to pull away the whole layer when you lift it up.
When you do lift it, it will pull away some spots but mostly leave a pitted surface in your base - like old concrete. This mixed with the uneven application of filler, should give you a realistic surface effect. You will also see in the thumb with the sandpaper, my shape test I made out of tracing paper, that I laid down on the wooden base before starting, to see if the idea looked like what I wanted. You may want to do the same use if you are thinking of a similar edge.
STEP 2- grey wash
I started with an oil based enamel wash as it is the opposite to the filler base medium (water).
MIG GREY WASH proved to be just the right tone I was after to get me to a starting color on the base that was grey brown. Just apply it with a typical medium brush, making sure that you follow the pattern you created as you laid the wash down.
Each aplication will get darker and the more you have in the brush the more it stains the initial point of contact. This can be difficult but if you are careful you can use this to your advantage and create a nice uneven covering of lighter and darker tones that follow the square lines. As you can see here the base has been taped off with masking tape and MASKOL has been applied (with a brush) to the exposed wooden edge up to the filler. This is for if you wish to do any airbrushing (as I was originally going to) but i ended up just using washes so it only acted as a spill protector.
Step 3- concrete colouring
This would have been where I load up my airbrush with Acrylic and start spraying some soft, square edged patterns in a dirty olive drab (light) but I decided to see where the washes would take me as I figured if it didn't work I had everything in place to save it with some airbrushing. I added some 'MIGMENTS' next instead. Concrete + light dust mixed with the fixative. I thinned this mix a lot so that it would only give a 'greenish' stain. I took a chance but the sensible thing with pigments is to always do a test first. I've never been sensible though.
Again, follow the patterns on the base that you created at the start.
Step 4- light dust/dirt
Next was to create a really thin wash with brick dust plus a smaller amount of europe dust. Apply this unevenly and mainly to the pits and holes in the base. It will dry quite light in color and be a good simulator for built up mud, dirt and general dust that has dried there over time.
Step 5 Final touch- darkershadows/stains/ light grey color
This step is not needed if you are happy with the results up to this point. Some will be.
I wanted to accentuate the shapes in the concrete a bit more plus add some deeper dirt/ mould colors as this was an indoor floor with only one window in the room. I added these with oils used very dry and lightly. Burnt sienna, yellow ochre + white and some greys.
From here you can add whatever detail you like depending on your subject. Mine will get some more dust and oil patches once the vehicle is in place. It is just as important to blend the vehicle and the base in together so that they appear as one and not as though it has been 'brought in by helicopter'.
I hope this gives you some ideas and help with a different kind of base.