Visit the Pacific Coast Air Line Railway
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The Pacific Coast Air Line was originally built as a 1/4" scale railroad, On30. I switched to a smaller scale standard gauge, OO(4mm/ft) slightly larger than HO(3.5mm/ft) and needed to adjust some features on the original 4x8, The most dominate O scale feature was the one tunnel portal. I decided to replace the original portal with an HO timber portal.
My original tunnel portal was an S-Scale portal made of urethane. It was way too tall for the new OO scale 1870's railroad that the Pacific Coast Airline had become. The timbering was way too large. I was originally going to scratchbuild a new tunnel. While at the hobby shop I found a HO Woodland scenics tunnel portal. It looked like the old portal so it followed me home.
I removed the old portal with a knife and chisel. The old tunnel liner was salvageable. It had the old O scale timbers but in the dark tunnel they still looked good.
The Woodland Scenics portal dwarfed my small 1870's engines. I cut off five boards from the bottom. D-oh, now it was too low and the eyebrow was too high
I hacksawed the top panel and three boards from the eyebrow. The crown was right. Making up for the original five board blunder I took two boards off the cut pieces. It was now the right size. The top piece broke but it was a fracture and fit together.
The tunnel pieces were glued together with Elmer's white glue.
Handling cause some damage in the pieces.
This is floor leveler, it is Hydrocal or equivalent. I used it to patch the damage.
Mix the plaster in a small bowl, add the water to the plaster. Wet the area where the plaster is to be applied.
Apply the plaster with a artist's spatula. I got this plastic one at Walmart. Build the areas up with excess plaster.
Carve the plaster patch with a hobby knife to match the original casting.
I prime the plaster with Kilz2 acrylic primer. This seals the plaster. Apply it in small areas overlapping the next while the areas are still wet. The primer makes an more controllable surface to stain than the plaster.
Yellow arrow unprimed
Blue arrow primed
Sepia drawing ink stained

Sepia drawing ink makes a good newer wood.
I used an India Ink and alcohol stain. Mix the WATERPROOF India ink into the alcohol bottle until it produces a stain on a paper towel about like that.
Paint on the I/A solution onto the casting. If there are any spots that won't stain well apply a small amount of undiluted India Ink to the spot.
Highlight the bolts with burnt sienna. Then spray it with flat finish. I like Floquil's.
Fit the portal on the layout. Mine is on a curve and requires clearance adjustments. I took my longest car and ran it through the tunnel.  Allow more clearance at the blue arrow. Mark the location.
I used a newspaper mask drawn tight with a map pin and a heavy object. I chiseled away the old ballast so the portal was flat.
Layers of strata were carved away. The portal was hot glued to the benchwork and the interior to the portal.
A piece of strata from above the tunnel was used to make a template.
The template helped with maintaining the angle of the strata up the side of the portal to the fascia board. The vertical lines are the width of the opening. The ceiling tiles were cut using a knife.
Click here to go to the original tunnel construction.
I glued the tiles up the fascia board side. The tiles were notched to go around the interior box. The last tile at the top of the tunnel was thinned so it would line up with the top of the portal. It is hard to get the ceiling tile pieces properly "jaggedy" in this small area.
Click here for more on rockfaces using ceiling tile
Holes can be filled with small pieces of broken tile and a rougher face can be made with small pieces glued on. I don't remove the white or brown layer until the tile pieces are in place. Just flick them off with a knife.
Using the template from the original piece allowed me to keep the same strata angle. I used a large piece from the original rock formation.
Spray the rockface with Awesome household cleaner.
The Awesome breaks down the tiles and causes them to "stratify". I couldn't use enough Awesome because of my stream bed. I painted the whole face with Kilz2 primer to seal the tiles and then a coat of base color.
I used burnt and raw sienna to try to match the original rocks in the strata line at the blue arrow. Use a small brush to add highlights with the India Ink/Alcohol solution.
The tunnel portal came out quite well. It took a finish and looks like weathered wood.
Click here to make non-wood materials look like weathered wood