This method is great for making a batch of signs in bulk, and means that the same sign can easily be reproduced at a later date. It involved applying paint to engraved areas, and then sanding back the unengraved areas to reveal the sharp edges of the engraving. This example shows the production of three sandwich boards for the Tchai Ovna House of Tea in Glasgow.
Prepare your vector artwork for laser engraving - you can use this as an opportunity to mock up the final look of the sign by playing with fill colours and superimposing onto a textured background, as I did here. I was using 18mm spruce ply which was to hand.
If you would like to colour the background of the image, now is the time to do that. In this example we were going for a bare timber background.
In areas of the artwork where two different colours border each other (eg. in the 'V' of 'Tchai Ovna' in the picture), remember to design in a thin raised 'wall' of unengraved area, to mark the boundary for painting later.
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Step 1: Laser Engraving
Engrave all areas that are to be filled with colour, to 2mm depth. We used masking tape to engrave through so that most of the fume marks that occur with raster engraving, took to the tape rather than to the timber. Once engraved, we removed all masking tape. You may want to leave the masking layer in place, particularly if you are using sprayed paints, as described in this instructable.
Step 2: Painting
Apply a little furniture wax to the borders of all engraved areas to reduce the ingress of paint here. Apply the paint to the engraved areas. You can do this quite liberally and roughly. I did this by brush so that I could apply three different colours simultaneously without having to re-mask any areas. Allow to dry fully.
Step 3: Sanding Back
When dry, a light sanding is all that is required to remove the excess paint and reveal the sharp edge of the engraving again. If using powered sanders, progress carefully to get an idea of what grade of paer to use and how much time to spend on each area (120 grit worked well for me). Clean away all dust thoroughly using a vacuum cleaner.
If you accidentally sand away any areas of paint, simply touch them up again as required.
Step 4: Finishing
Finish the material as required. For our purposes, outdoor sandwich boards, we used wood preserver and yacht varnish.