I recommend starting any project with a test grid. For a fill test, though, you'll need to modify the grid, essentially using it twice—once using all the same settings on all engraving areas and again with the settings you've annotated on the axes. Start with a base test grid. I used an M1 settings test file from xTool (that I can't find online anymore) and removed all the cuts from the test.

Set the axis labels to represent the setting you want to try for the powder fill. I found you need something mid-speed and pretty low power. Then select all the test marks themselves and set them all to the engraving depth you plan to use and eventually powder fill on that material. They should all be the same settings.

Place the wood you wish to test in the machine and afix it with tape so it will not move between processing runs. It's very important that it remains in place through the entire test.

Frame and process this job. DO NOT remove the finished engraving from the machine—just leave it in place for the next step.

Now it's time to apply the powder paint to the surface while it's in the engraver. Do not remove it from the machine—just do this in the machine while it's still in place. I use Pro-Tec powder paint, which is designed for fishing lures. A very little bit goes a long way. Notice in the picture the tiny spoon I use to spread some across the board. Then use a semi-rigid straight edge to smooth it into the engraved areas. Following the advice of many online tutorials, I've come to use a metal business card. The long edge works perfectly, and the flexibility of the cards make it easy to maneuver around the design. Continue to smooth it out until the powder paint is flat across the engraved area. Move any remaining powder to the edge or scoop it off and return to the jar.

Clean up around the area with a moist cloth, and remove anything foreign from the engraving field. 

Next, copy the test grid and labels to a new canvas. Copying and pasting in XCS between two canvases now retains the objects' original locations, so you can be assured they'll be in the same location as before. Don't forget to set the thickness of your material with auto-measure.

This is when you're going to use the settings engraved on the test plate.

Column by column, select the test elements and apply the power level indicated by the labels. Then, row by row, select the test elements and apply the speed setting indicated by the labels. 

So you don't end up re-scoring the reference labels, select the layer containing the scoring for the labels, and set everything to Ignore. They're just here as reference.

When everything's updated with the indicated settings, frame and process the job. Do not use air assist in this run if you have it. The powder paint is very lightweight, and it will already spatter a bit. You don't want to blow it all off the engraving surface!

When the job is finished, you may want to cut the test out to use it as a reference. I keep a catalog of these plates in my workshop to refer back to before each job. I included a canvas in the sample design file that just includes the frame for cutting out the test from the board.

Remove the test from your machine and determine which combination of settings looks best. You may need to revise the test grid and retest if you're not seeing the results you expected. Notice that this grid I'm showing is different—more granular—than the first test I ran. I want to really dial in the settings to get the best effect.

For me, 160mm/s at 6% power seems to be the best combination.

But keep in mind, that's the best combination for this particular paint brand and color, on this particular material, engraved to this particular depth, with this particular machine. Change any of those factors, and the results could be different.

Conclusions

So why go to all this trouble? I've seen some amazing jobs done with powder fill, and I'm hoping to use this process myself on future projects.

Sure, I can always do the masking thing like I did on this sign project, but I'd like to get to a point where I don't need to use my heat gun to finish the project.

If you've perfected your powder fill process, please share your settings and results here, too!