This week’s project was about both plasma cutting and finishing metal. While I was excited to utilize the plasma cutter, I was equally excited to investigate finishing techniques. My goal was to answer the question of what is the best way to paint and protect metal?

Step 1: Creating the base shape

Before I could do anything relating to finishing techniques I first had to cut the diamond shape. I decided against cutting an inner shape because my primary investigation would be in finishing techniques. To create the shape I first plasma cut the shape, then used an angle grinder to take the excess slag off the edges. This create a grooved pattern in the steel (see image on left) that would create issues for me later. I also took a file and rounded the edges of the shape by employing a 45-90-round filing technique. While not perfectly round, the sides were safer to touch than the unfilled edges. The image on the left is of the unfilled edge.

Step 2: Preparing the metal

Now that I had the shape I wanted, I needed to prep the metal base for paint. Part of my investigation was looking at how paint attached to sandblasted vs. sanded metal. Ideally I would also have a stripe of untouched metal to compare the three, but with the angle grinder leaving its own grooves in the metal this wouldn’t be possible. Therefore I opted to use a power sander to sand the entire surface in one direction to try to eliminate as many variables as possible. Once I had sanded the full surface, I taped two sides so only the middle would be exposed for sandblasting.

 

Once sandblasted I removed one of the sides of the tape. This way I had a sanded and sandblasted surface to paint. I left one of the sides covered so I would have a control surface to look at how a protection glaze would work on an unpainted metal base surface. I then covered the right side with an additional tape strip to leave the reference surfaces intact.

 

Step 3: Painting

For the type of paint I choose to use a standard spray paint. I didn’t want to introduce the variables of brush strokes into looking at how the spray paint was applied. I applied the spray paint in thin layers to allow for quick drying and an even coat. I applied two layers in total, using 220 grit sandpaper to sand down in between the two layers. After the second layer I used the finest grit sandpaper to create as smooth a finish as possible.

Overall I didn’t find too much variation in the way the paint stuck to the sanded vs. sandblasted surface. Once two coats were applied there was very little dimpling in the surface from the under layers. On one of my “test” diamonds I tried adding additional layers of spray paint (up to five layers) and could not eliminate the dimpling effect. This lead me to conclude that perhaps the steel held a version of the dimpling that would be present in additional layers of paint. When I was investigating my finishes I also looked into the type of paint to see if it would not have these effects.

Step 4: Protecting the Paint

One of the elements of the question I was looking to answer was how to protect the metal and paint from scratches. For this I wanted to look into using a clear coat vs. a lacquered paint to see if there were differences in their level of protection (see image left).

I took two of my sample diamonds to test the protection glaze on. The diamond on the left had a clear glaze and the diamond on the right was a lacquer (side note: I couldn’t find a clear lacquer so I applied a colored version). My goal was to perform a scratch test across the three bases (control, sandblast, and sanded) on both finishes to see how they performed. The least scratched overall was the clear coat on the control surface and the least scratched painted surface was the lacquer on the control surface. If I was to repeat this experiment, I would want to try a just a lacquered paint on a sandblasted surface. But since I was looking into how to protect a regular spray painted surface, I went with the clear coat on my final two pieces.

Step 5: The Result

The final result is shown on the image on the left. After putting a clear coat on the paint, I removed the tape to show a comparison between the paint & coated right hand side and the original finish on the left. I then typed up a diagram on the back to show how I did the finishes overall.

Conclusion:

If I were to restart this project, I would recommend doing the same cutting techniques, sandblast the surface after angle grinding, and using one coat of a lacquered paint in the color of your choice. The reason I would recommend sandblasting is that though the sanding and sandblasting held paint the same way, the sandblasting was a little less destructive to the steel (i.e. less grooves). I recommend the lacquer for the paint because on my test pieces the lacquer had the least amount of destruction.