By Emilio Renes
Modified and translate from the original article in Panzernet

The photoetch is the more frequently used complement in model making, is one more older technique than photography, but its use for the obtaining of "printed" solid pieces has its origin in World War II, when were constructed the aim sights of the airplanes using this technique.

 It is not the objective of this article to create an alternative to the existing photoetches in the market (Eduard, Aber, etc), since made impossible to obtain its quality with the methods that are going away to describe, but is interesting have a tool that will allow us to create our own pieces with characteristics that never could be obtained with other scratch techniques (grids, clasps, zippers, complex panels, etc). On the other hand, the artisan aspect allows to make figures that would not be able in a commercial photoetch, at the cost of certain patience.

Necessary materials

1.  A decent PC and a good vectorial drawing program (Corel Draw 8,0 or better, Adobe Photoshop, etc) Its essential that the    program be able to draw with exact scales.
2. A printer with a resolution of 600 dpi or better, laser or ink jet. The printers with inferior graphical resolution to 300 dpi produce a disagreeable effect of stage in the oblique lines when they are very small.
3. Paper adapted to the type of printer. The paper with the ideal impression would be completely opaque to the light where it is the ink of the drawing and completely transparent in the white zones. In the case of the ink jet printers, the transparencies of high quality reunite these characteristics, although the ideals are very expensive (the best transparencies are of the Epson or Pelikan brands). When was laser printers, the acetate sheet can be useful, but be careful, when the toner level is low, if we watched with backlight a painted zone of black in an acetate sheet we observe very small holes, which can ruin the final result. The solution consists of altering these zones with a black ink marker, or diminishing the time of exposure (to see more ahead). Another alternative with the laser printers is to make the drawing in vegetal paper or normal paper and impregnate it with paraffin oil or something that makes it translucent and does not stain excessively.
4. Metal sheet. The photoetches can be done of copper, brass, zinc (better is not), nickel or aluminium. Habitually we will use sheets of brass/copper of 0.1 or 0.2 mm. The 0.1mm sheet is possible to be found to a disturbed cost in certain stores of artistic materials (u$s 12/m). The 0.2 mm sheet can find it in specialized metal warehouses and most cheaper.
5. Photosensitive lacquer. The photoetch principle is to cover the metal with a substance that in contact with the ultraviolet light becomes soluble to a revealer, being on the zone nonexposed an identical image to that we have drawn, and that is resistant to the "mordant". The photoetches are used in serigraphy and the preparation of printed electronic circuits. Perhaps the bravest dare to create his own photosensitive lacquer with ammonic dichromate and fish glue, after an exhaustive erudite book investigation. But the easy thing is to use a photosensitive lacquer thought to make printed circuits, like for example the Positiv 20, that can be found in stores of electronic components. Perhaps the salesman says that is not used already, because the plates are sold already printed, etc., etc.
6. Revealer: In the case of the Positiv 20 is used a solution of Caustic Soda and water by 15 grams by liter (it is better than 7 gr./l, as says in the prospect).
7. Ultraviolet light source. If we are not going to gain the life doing photoetches, it is not necessary to buy an ultraviolet lamp of  u$s 2.400 nor a mercury lamp. The solar light is better than any other thing. If is cloudy it is necessary to increase the exposure.
8. Mordant: forget the Dutch mordant, Nitric Acid, Mixtures of H2O2+HCL, etc. The best thing is the Ferric Chloride at 40% (400 gr./liter), since is a noble mordant, of foreseeable and little toxic effect. The disadvantage is that is very dirty (if we did not know that it spoils the paint would be ideal to imitate oxide and rust). When the Ferric Chloride is saturated with copper salts is possible to regenerate it with strong water (HCl) at 30%.

Hands to work

1.       Draw the photoetch sheet in the computer. Verify well the measures, the scale, etc. The drawing must be in black and white. The gray tones does not serve in the photoetch, although there is something similar in serigraphy techniques that are called plots, but whose final result is unpredictable in the subject of which we spoke. I recommend to print a rough draft and imitate the final photoetch as if one was a trimable one to verify if the measures are correct, etc.It is necessary to assume certain laws in the design. The "holes" or "lines" with an inferior thickness to the sheet thickness never excavate the metal completely, but this can be useful to imitate panels. If you make a grid, the strips must be a wider 20% of which they want that they are, since a certain component of lateral corrosion exists that will thin the strips more of the predicted.

2. Print the transparency with the photoetch draw in high quality print.

Figure 1. Sheet printed in adhesive transparency for ink jet printer.

3. Prepare the metal sheet to give the photosensitive layer. Wash it with Ammonia to avoiding the fingerprints and the fat spots.
4. Give it the photosensitive layer. In the case of Positiv 20 sprays, is recommendable to follow the instructions of the manufacturer. It is necessary to apply a tenuous layer in zigzag, locating the spray at 20 cm and balancing smoothly until obtain a uniform layer. All this must be done in poor light, avoiding the solar light. Let dry 24 hours or more, or can be put in a normal oven during 20 minutes at 60ºC. Take care of not ventilate well the kitchen, since it has a rare and penetrating scent.
5. Exposure of the sheet. It must stick the transparency to the photosensitive layer without left hollows. For that reason the sticky transparencies are the ideal. Another possibility is to use a glass sheet as weight. The time of exposure not have fixed rules when the solar light is used, but with 30 seconds under direct light usually is sufficient. There is people who use a 200 watts incandescent light bulbs (the present halogen lamps are ecological and lack of UV rays): I recommend to test the exposure until dominate this technique.
6. Revealed . When revealing we will appreciate the failure or the success. If the image vanishes, is that the exposure has been excessive. If it takes much in appearing the image (more than 2 minutes) and mainly, if the brightness of the metal in the zones exposed to the light is not appraised, those that do not have drawing, it is that the exposure is insufficient. Much more treason is the nonapparent sobreexposure, being left in the black zones an invisible itch by which the acid will act and the photoetch will fill of little holes. Never use an excessive concentration of revealer when this is NaOH.
7. Wash the rest with water, but not rub them with brushes or abrasives.

Figure 2: Brass sheet after make the exposure and revealed.

8.  Cover the back part of the metal sheet with nails enamel or lacquer, that avoids the action of the mordant. Or we can leave it without covering and expose it to the mordant, which will diminish the thickness of the plate, assuming the risk of which were not homogenous.
9. Submerge the sheet in the ferric chloride solution. The following advice justifies this article, since all the previous one can be found in bibliographical references: The side with the photoetch drawing must be horizontal and watch to the bottom of the container, but never to lean in, that is to say, it must float down. It can be obtained with cork covers or polyurethane as floaters. Of this form, the insoluble copper salts fall to the bottom and the speed of engraving will be homogenous. Of another form, the copper salts can inhibit the reaction locally, and thus we find zones excessively deep next to incomplete zones. Watch the process periodically. For a 0.1 mm sheet are used approximately 2 hours in obtaining a complete engraving.

Figure 3: Seen here are the photoetches of the M3 Stuart gasoline version grid and the Heller AMX-13 ventilation grid.

10. Clean the rest of lacquer with acetone and the photoetch will be ready.

3D Effect
To obtain a three-dimensional effect, with prominent zones like screws, etc., demands to make 2 sheets and repeat the photoetching process by duplicate, being left protected these prominent zones (would be only drawn the screws) and producing an incomplete engraving that thins the rest of the plate. Soon one becomes to print the sheet with photosensitive lacquer and protect all the drawing, making the complete engraving. Perhaps exists alternative methods using plots and decreasing concentrations of mordant, etc.  

Photoetch by both sides
Is suitable, but the difficult one to be able having to align two specular images of both sides of the sheet. Both drawings must be exact and adjust exactly to the sheet with identical margins.

Photoetched in aluminium
Equal to the described above, but be careful to reduce the concentration of the mordant, that must be of 20% or less, so that of another form produces an intense exothermic reaction.