¹⁄35

Sokol

First, I had to understand the whole thing of homemade photoetching. You have to find a way to print a black scheme on some copper, dip the tinted copper in some FeCL3 bath, and the acid will basically attack the metal where it wasn’t tainted.

Most of the material can be found at okay prices in electronic shops as this method is also used to create printed circuits.

The first step was not really hard to do as far as I was concerned. I used some graphic software to draw all the pieces I needed. (pic 10)

Those included the holed plate under the gun but also the side plates, stanchion supports, supports for the gun cradle, funnel parts, access ladders etc.

The thing you have to remind yourself is that the less metal the acid will have to eat, the less your acid will be clogged. Which means you will be able to use it several times afterwards.

So I absolutely crammed the drawing of the fret with various wheels and stuff I could need for later.

Of course I used some kind of pen instruments available in the software to tie each piece to every one in order for these not to fall in the acid when they would be etched enough. I then put some big side borders because you have to be able to handle the fret between the different operations.

When it was done, I used a laser black and white printer to print the future fret and went to a photocopy shop to get 2 copies done on transparent celluloid (I know there is some special transparent paper available directly for the printer too but those don’t come any cheaper).

Now the thing you are supposed to do is to spray a small metal fret (copper or brass, 0.02 inch thick) with a photoresistant spray on both sides and then expose the metal covered with your celluloid sheet to the rays of the sun for a few minutes on each side. Then to go in a dark room to dip the metal fret in a revelant bath until the pattern you want to revel comes in.

That’s the bloody theory, the very same as if you were about to develop your film by yourself. It certainly works for printed circuits maybe because you can afford getting a dirty job but not when it comes to model making.

I just couldn’t manage to spray the metal evenly –and then as the photoresistant takes several hours to get dry, a single grain of dust falling on the fret would attract the liquid and completely spoil the whole job.

I tried this way several times, getting more or less bad results, which led invariably to some Frankenstein like bits once I dipped them into the acid until I made the decision to buy some special recto verso photoresistant brass that is sold on railroad modelling mail-order shops.

The stuff comes as a fret covered on each side by an opaque plastic film and is not so cheap.

The following step would expose the fret under the light of the sun with your pattern as a screen for a few minutes so that the photoresistant material is getting activated. I used the simple trick of putting the fret in sandwich between my 2 celluloid sheets and to actually use superglue to attach the fret to the celluloid –this way I was pretty sure that the 2 sides of the fret would be corresponding. Now the problem is “How much exposition time”? I have been reading 2.5 min under full sun.
Duh and what about winter sun?!

I let 4 minutes each side under French June sun and it worked, up to you to work this out.. (pic 10)

Well, the problem is that there is no sign your fret is supposed to be ready until you developed the stuff... (pic 11)

Anyway, you have to go in a dark room (because your fret will still be sensitive to light until it’s been fully developed!) and use a plastic container (I used the kind of things you put under plants) and then to mix the right amount of water and developer (refer to the documentation). The problem is that the mix should be at a temperature of 25°C to really work, and be constantly agitated/stirred. Once I used a mix that was too hot and I saw the pattern develop extremely fast and then dissolve completely. So you have to be careful and find a way to use the right temperature.

Just agitate the liquid gently and then you will see your pattern appear. When it does, just wash the fret under clear water to stop the developing process.
Then dip the fret in your acid.

And be very patient. I know there are some ways to speed the process using very aggressive chemicals but those may be very dangerous so I won’t cover them. Just let a few hours and try to agitate the acid regularly so that it bites the metal okay and work better.. I succeeded with the first fret on my first try once I had bought the already photoresistant brass sheet.

I tried to do another fret a week later and somehow did one or two failures but those proved fantastic to model some rusted parts. (pic 12)