I am ready to apply color to a woodland scenic portal and retaining walls. any advice on sequence of applications and types of applicaions? i have a variety of polly s paints and woodland earth colors (concrete, umber, and ochre), and a variety of inexpensive acrylics that i generally water down for various uses. any need for airbrushing? comments are welcome.
I used the WS concrete stain for the base color on mine. I used a brush dipped in water to thin it down. I didn’t seal my castings first because I like the way the stain penetrates the plaster and gives and uneven effect. I then used a couple of the darker WS stains for high lights. It’s really EZ to do. If your going to seal them afterwards, make sure they dry completely before you dull coat them. (like a week) The dullcote reacted badly with the still wet water/stain, and turned white.[xx(]Had to redo one.
Hi Mr.F: FWIW, here’s another technique you may want to try. This will work on “raw” plaster or pre-colored portals and retaining walls. Lay them down flat, then spray them with a rubbing alcohol/India ink mixture. Let dry, reapply if it’s not dark enough. When you are satisfied with the results, dry brush with white or light gray acrylic paint. Just my opinion. Here’s an example. This is a stone arch bridge, but the same technique would apply to a tunnel portal. 
I used El Cheapo acrylic paints from Wal Mart. I used a teaspoon or so of grey as a base, added one drop of yellow, a drop or two of Burnt Umber, and then made it into a somewhat heavy…ish…wash. Once I had painted the portal with one lash of that stuff, I used a black wash of acrylic paints to make the crevices stand out. Later, when it was good and dry, I used a heavier wash of the black to make the smokey look at the apex of the arch.
Welcome Aboard!
I have tried a couple of different methods, but the one I like best now is to use Rustoleum’s speckled paints, particularly the one in light brown. This gives both an uneven texture and a lot of color variation.
(Click on the picture for a larger view.) I did not treat the casting beforehand - just sprayed on the paint. After it was dry, I used a thin solution of India Ink in water, and brushed it on. I do that to highlight the deep spots and cracks in the casting. Sometimes it takes a few applications of the India Ink mix, but usually two is enough.
Used an airbrush with Woodland Scenics concrete that was diluted with water. It went very quickly. A total of about 8 feet was done in hr. Two coats. First coat was absorbed quickly so that by time I was done first coat the second was started. Some additonal weather ing will be done 
I use acrylic craft paints. First, I paint the entire casting dark gray. Then, I drybrush on lighter grays and browns, and occationally some red or orange until I get the look I want.
Nick
model-trains-video.com has a series of how-to videos for sale. Volumes 4 & 5 cover scenery construction and in Volume 4 Joe goes over building a tunnel portal including painting and weathering. Good stuff.
And even better, you can download the chapter on doing the tunnel portal without needing to buy the whole dvd! I’ve seen the portal that was done in the video and it looks great!
Regards,
Charlie Comstock
This is how my portals turned out using the acrylic washes.
-Crandell