I had to try it to see for myself. Actually the aim of the experiment
was three-fold:
1) To see how well FeCl3 etches aluminum
2) To assess the properties of my new home-made resist
3) To assess the properties of the new back-spray method

A small 2' x 2.5" x 1/8" aluminum plate was sanded down on one side
to 400 grit, 220 on the opposite side. Cleaned with acetone. Resist
applied and some Roman numerals carved in as a pattern. The plate was
then back sprayed with two coats and allowed to dry overnight.

A small amount of commercially available FeCl3 solution, used may
times previously on brass and steel, was placed in a Pyrex dish and
this in turn placed on a dinner warming plate dedicated to that
purpose (I usually try to get the solution to about 35 degC).

Results.

1) Nothing much happened at first. After 20 minutes the solution was,
however, boiling lustily. Adding water made no difference, so the
plate was removed and rinsed in water. This stopped the boil and the
solution was immediately neutralized to pH9.

2) The (rather soft in the first place) resist all but melted and what
was left was easily brushed away.

3) The back spray partially peeled off, the rest was bubbly as one
would expect if significant heat was applied to paint.

4) The etching result is he

http://www.flickr.com/photos/2768312...7622484352534/
http://www.flickr.com/photos/2768312...7622484352534/

The interesting effect which I have seen in someone else's work but
never in mine is the deeper etch at the edges of the etched area where
the bare metal meets the resist. I have confirmed with a magnifying
glass that the etched lines are deeper at the edges with a slight
ridge in the middle. On the back it is even more pronounced with a
trench running along the border where the back spray paint peeled off.
Anyone knows what this is due to? I suspect that gas production is in
some ways to blame but I cannot guess at the mechanism.

Conclusions:
1) The new resist is useless (actually I knew that before the etch -
it is too soft).
2) No conclusion can be made about the back spray method - not many
will withstand boiling temperatures.
3) FeCl3 does not need much help to etch aluminum, unlike copper,
brass or steel.

Michael Koblic,
Campbell River, BC