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What about machinist's layout fluid? I've painted some on brass and it laser engraves off quite nicely. For example, Dykem blue layout fluid (www.mscdirect.com, item 00264036). But is it thick enough to prevent etching? It certainly dries quickly, but might be hard to build up a thick layer (at least with the paint on--perhaps the spray would work better?).
What software are you using to convert the Gerber?

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I've been milling circuit boards on a small CNC mill. It seems like you should be able to do something similar on the laser cutter, but with much more precision. And no etchant or spraypaint needed! Instead of removing unwanted copper you just burn out the spaces between the tracks. You save time and energy by not having to remove all of the unwanted copper. An example via a GIS for "pcb mill":

http://www.tekit.seriouslyinternet.com/gcodepcb.jpg

I'm pretty sure the lasercutter can remove the layer of copper, can't it? Or maybe I'm talking out my butt.

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Okay stay with me here a sec....

There are two things that I wonder.

First, if you could adhere a thin enough sheet of copper to the board you are trying to make, I wonder if it is possible to spray a water soluble substance like the CerMark spray on it, burn away what you dont want and wash away what you do want(not sure if a standard laser cutter like you have will burn it away, or how thin the copper can be to produce the desired affects?!?)

Second, what seems like it could be developed would be something like T-Ink which would hardened when in contact with a laser (think stereolithography and how it works with resin). That way you could develop a (semi)liquid that you could put on a board that would harden with the laser. This is probably a little more advanced for immediate purposes, but it could help speed up how the boards are made.

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That's way cool. Ok, so here's some ideas:

1) Dgrover suggested layout fluid. I second that idea.

2) I suppose you could just paint the entire board with permanent marker, everyone knows that makes a good resist. :)

3) Here's a really oddball one. Electrostatically coat the board with powder-coating material, and then use the laser at a lower power level to fuse it to the board where desired. Heating the board to just below fusion temperature might help. I have no idea if this is actually possible - but if it works, you could use the same process to apply powder-coated silkscreen and resists too!

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I assume your laser is not powerful enough to burn the copper directly, otherwise you'd have gone that route from the start? maybe try using 1/2 oz copper clad instead of the common (heavier) 1 oz?

or here's an easier method for resist... rub your bare board with a block parafin wax, all you need is a thin coat. then use the laser to burn away areas that need etching. I'm not real familiar with a laser etcher, does it heat the workpiece, or is the laser so fast the material is vaporized without transferring any heat to the workpiece? obviously with too much heat, the wax will self-heal the burns.

yet another idea would be a photo resist board - just leave the protective plastic on and have the laser burn away the plastic and the thin resist layer under it - then straight into the etching bath.

to help with any of these methods, and just for pcb making in general, when you're drawing the layout use the copper pour or polygon command to fill in all the voids between pads / tracks, so you don't have all that open-area that the laser needs to uncover and the etchant needs to remove.

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I took printmaking in college and we used asphaltum (a quick-drying liquid form of asphalt you can get at places like dickblick.com) to mask-off the areas of our plates we didn't want to etch. I don't know how well a laser would remove the asphaltum, but I suppose it should do alright.

Our instructor had this cool wooden, open-topped box with a spinning turntable in the center that we used to apply the asphaltum. We'd hold the plates onto the turntable with thumbtacks and switch on the turntable. Then, while it was spinning, we'd pour a bit of asphaltum onto the center of the plate and the centrifugal force would sling off the excess. Very cool.

Asphaltum cleans up with mineral spirits after you're done etching. Hope this helps.

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Try using a selenious-acid based cold blue, like Brownell's Oxpho-blue, to create a dark oxide coating on the copper.

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drepetto -- your comment is on the money - I've used laser cutters to do that exact process before, and it works like a charm. I'm not sure that their laser cutter has the power - - I was using a 170watt dual tube cutter.. but the power and PWM was turned way down..

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Watch OUT! Anything you laser off of the boards will be ionized, potentially catalyzed, and not all of the vapor will be captured by the laser's filter. Not only that, many substances (such as chlorine-containing compounds - think PVC) should never be cut/etched in a laser etcher due to the corrosive nature of the resulting vapor. MSDS for the compounds you are working with will NOT cover all of the risks of vaporization, as the temperatures involved are much higher than the heat of combustion. FYI - bologna is a really bad thing to cut in a laser as well. You just try to get out the smell...

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I'm not sure what's different about a standard cnc laser cutter and, say, what LPKF sells. Is the black stuff really necessary? We just got one of LPKF's CNC mill circuit prototypers at work. While the software isn't fun, the thing has incredible spatial resolution and we're using it to machine rf and microwave boards where we need to control the impedance accurately. However, I don't know that any of these solutions is really within the realm of the common hobbyist.

Do you all really have this kind of equipment available for hobby stuff?

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First, I would agree with solderer above. Copper compounds can be extremely toxic, and you should be careful to vent the fumes.

Other than that, I remember working a job where we sprayed powdered graphite on some stainless steel tubes to laser weld them. So, you might simply try dusting it with graphite powder. Anything that will absorb the energy from the laser should work. The question is what will easily wash off afterwards.

Just my $0.02...

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For those that are wondering why you don't just burn the copper off, there are many reasons.
If a medium power CO2 laser, it won't even touch the copper. That wavelength and copper aren't too compatible.
But even if it were powerful enough or something like a Yag laser, copper is harder to go through than PCB material. Once it goes through, the board gets burned too. It's very hard to depth limit a laser for something like this unless a special type of board and laser are used.

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