anelok: the Y-Box paint job, part 1: masking

Werner Almesberger werner at almesberger.net
Sat Nov 16 13:02:10 EST 2013


I've now pretty much finished the Y-Box cases. There were a few small
bugs in the design and I had to tweak the mill setup some more, but
I finally got enough usable case parts to move on to the next step:
painting.

The general idea is to paint the case from the inside with acrylic
(water-resistant) paint. This will preserve the perfectly smooth top
and bottom surfaces of the acrylic, the paint will be protected from
scratching from the outside, and it's easy to make "windows" (e.g.,
for the LED in Y-Box and later on for the much more demanding OLED
window in Anelok.)

Drawbacks of this approach include the more complex inside geometry,
that you can't paint over scratches, and exposure to scratching from
the inside. More about this later.

In order to control where the paint goes, I have the choice between
working with the tiniest of brushes and a very steady hand, or an
airbrush and masking. It should be obvious which approach I
picked :-)

For masking, I get to choose between masking tape and masking fluid.
Here's what the results of either look like (from a preparatory
experiment - these case parts have some small flaws):

http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-paint-mask-comp.jpg

The one on the right was made with masking tape. There's no way to
cover the small openings - the best I could do was to keep paint from
flowing to the outside. At some places a bit of paint crept under the
tape. The tape also gets in the way when painting.

On the left we have the result with masking fluid. Much cleaner. It's
more work to scratch/peel off the mask but the result speaks for
itself.

Here are the parts I used to make the final cases, with masking fluid
(blue) applied:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-paint-masked.jpg

I thought it would be nice to also keep paint from the case's edge
on the USB A side, so I "painted" a strip of mask there.

The next question is how to remove the mask. In the previous
experiment, I used a knife to lift the mask off the plastic and then
peel off little strips. Doing the same with the "good" part produced
nice results, as expected:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-paint-knife-strip.jpg

Since the masking fluid is water soluble, one should also be able to
wash it off. So I put the second case in the ultrasonic cleaner. This
is what it looked like after about four runs of ten minutes each,
plus some gentle swiping with a paper towel:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-paint-bath-strip.jpg

Most of the mask came off but there's still plenty left for the
knife, especially in corners. A bit of paint also came off at
places where it was supposed to stay. Not sure why, could be that
some masking fluid had spilled. Lesson learned: also the ultrasonic
cleaner has limits.

The paint surface isn't very smooth. Most of this comes from small
height differences left by the CNC mill. Some also comes from gas
bubbles. Seems that it wasn't such a good idea to race the sunset
when painting ... 

Anyway, most of these flaws are nearly invisible in real life.

Next: polishing.

- Werner



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