Wednesday, 27 December 2017

Murrumbidgee River Bridge - Part 3. 2nd span, and painting

Modelling can be very tedious at times.  Having finished the overhead beams on the first span, the second span was just a long slog on the workbench.  As I was now confident on my technique, the remaining girders were mass produced, and then fitted to the second truss span over a period of 3 weeks 

The Murrumbidgee River flooded last year - this is a picture taken of one of the billabongs during the flood, Note the gum trees, as this is some of the effect I hope to have around the bridge model.

Girders ready to be installed.  The above represents around 12 hours of workbench time

The instructions of the kit recommended dunking the completed bridge span into paint, and allowing it to drip dry,  Whilst  this had the advantage of speed, I have doubts over this method, as well as having sufficient quantity of paint.  In any case, the brass had to be cleaned of flux, fingerprints, cobwebs, and anything else that shouldn’t be there.

Rinsing under the tap was all I had done so far, but evidence of green verdigris in certain areas, showed that the tap  does not get rid of all the flux.    So the search started for a suitable container where I could scrub with a toothbrush, and use chemicals if needed.    Each of the spans is around 50cm in length.  To cut a long story short, (and no, the house bathtub is not suitable),  I made my own out of timber, and plastic film.  My inspiration is thanks to a display of R/C boats that I saw at the Newcastle model exhibition many years ago.  They had made a large shallow pond  on the floor of the exhibition hall using black plastic and bricks

The timber is scrap I happened to have lying around  

The plastic is double skinned to prevent leaks in case the upper plastic film is damaged.

Scrubbing was with cream cleanser, and toothbrush.  Followed by a rinse.  Then repeat.  Check for any verdigris, and if present, break out the ammonia and salt solution.  Wear gloves.  Rinse again.

After cleaning, the next stage was to attach the rivet shim to the upper, and lower chords of each span.  The shim needs to be carefully cut to size.  Checked for fit, I then glue the shim to the span.  Why glue?  Applying heat in the form of a soldering iron will cause the shim brass to expand, and then contract whilst it  cools.  I am not skilled enough to be able to control this, and any distortions in the rivet overlay will be difficult to correct.

Underside of the span, showing the rivet overlay.  Each of the main uprights are secured to the span with a pair of triangular gusset pieces made from scrap brass.

The glue I am using is 6 minute, 2 part epoxy from Great Planes in the USA.  Mix 2 parts equally, smear it over the bridge girder, and attach the rivet overlay.  Weight it down.  After about 10 minutes, remove the weights, and repeat on the next rivet section

Prior to painting brass, I like to provide a key for the paint with a light vinegar etch   The term used is “pickling”.  The vinegar is a mild acid, and the process gives a microscopic roughness that allows better adhesion of the paint.  My homebuilt container was half filled with fresh vinegar – about 1200ml.    As the bridge span is not covered by the vinegar, I am turning the model every 15 minutes to a new side.  You may notice a slight “pink” tinge on the brass after the pickling process, and this is a good sign.  After each side has been treated,   rinse the span and allow to dry

I am hoping that the vinegar treatment would allow me to skip the “self etch black” primer coat, and go straight to the final paint coats.  The self-etch black is not the easiest of paints to spray, and the bridge is rather a big model  The paint I am using is Floquil “Grimy Black”, thinned with turpentine, , and sprayed with a Badger 200 airbrush.   Pretty nasty stuff, so make sure that things are well ventilated.   Note. I know I should be using Xylene as the floquil thinner, but my experience with that product, besides expense, is that it dries far too flat. The turpentine , if anything, takes a bit longer to dry, and dries with a slight satin sheen.  And the other question is why Floquil?  Well, I have accumulated a supply of this excellent but now discontinued product, and to not use it would be rather sad, particularly as it sprays well onto brass, and gives a nice finish.

The main problem (other than the size of the span) with painting the bridge is the space between the 2 lattice etches on each side.  One had to try and avoid getting a shadow mask effect, so the airbrush angle of attack needs to be varied with each pass over the bridge.  My suggestion is that you do not attempt to complete the full span in one spray sitting – after you think you have painted the lattice, then allow the span to dry, and inspect for imperfections in a bright light.  Once you are happy with the lattice, then paint the remaining beams. 

One span painted.  I am happy with the colour, as it is close to the colour I remember on the real bridge.  The second span is still to get the pickling treatment, and then the paint. Note too, how the painted span only has 3 overhead support trusses. .  This is because the real bridge "shared" a support at the end of each span. .

The second  span should be a bit easier, as I had previously painted the insides of the lattice black (see part 1).  I will have to give the first painted span an additional coat, because, as expected in good light, there some areas where the brass colour  is showing through.  However, the effect of even this initial paint coat  is magic.  It is good to get a few wins on the way

The next part will be the wooden track support, and track.    

Until then, happy modelling

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