Porcelaine

My tiller bolts are shiny silver aluminium. I’ve always been slightly envious of the brightly coloured ones that Win&Win and some other brands sell.

About the paint

I bought a small bottle of Pebeo Porcelaine 150 paint in Lapis Blue about five years. It’s made for painting things like ceramic plates. I used it to paint the Penrose tiling spring gentians on this mug. It’s not super dishwasher safe but it doesn’t look terrible after years of use.

After painting, the instructions say to let it sit for at least 24 hours before baking at 150 C for 35 minutes. That’s fine for ceramics, but the strength of many aluminium alloys is quite sensitive to heat, and having a tiller bolt deform or fail catastrophically could be quite dangerous.

I couldn’t find any reports about using lower temperatures or shorter times to cure the paint, so I made some test pieces and tried:

150C for 30 minutes

120C for 30 minutes

In each case, I pre heated oven before adding the samples. After cooling, both samples had the same resistance to being scratched with a fingernail and felt smooth and glassy. In comparison, a piece that had sat at room temperature for a week was still easy to scratch with a nail and had a slight plasticky feel when rubbed. As 120C was clearly fine for my purpose is having a hard paint layer, I then tried the same temperature for 10 and 5 minutes. The 10 minute sample felt as hard as the 20 minute sample, but the 5 minute one was noticeably easier to scratch. For the shorter times, I used a thermocouple to keep an eye on the temperature and kept opening the oven door to keep the temperature constant as domestic ovens often aren’t terribly precise.

So, instead of 150C for 35 minutes, I knew 120C for 10 minutes was plenty to harden the paint. Would it be ok for the bolts though?

About the alloy

It’s impossible to tell what aluminium alloy something is just by looking or measuring the density. Looking online, some aluminium bolts are made from the 7075 alloy, which is one of the strongest. It’s also one of the most heat sensitive. In order to give the alloy its strength, it’s generally baked at 120C for 24 hours. This combination of temperature and time lets lots of tiny precipitates grow just enough that their size is the optimum for preventing plastic deformation. However, if it’s heated for longer than that, the strength begins to decrease as the precipitates get too big and too far apart, which is called over ageing. If my bolts were made from this alloy, then I could be pretty confident that if I painted then and then baked the paint as directed, they would be somewhat compromised. The chance of this causing failure are pretty small, but I wouldn’t want to risk that. However, using the shorter curing time at 120C would be fine though as an extra 10 minutes is well within the margin of error on a 24 hour heat treatment.

I contacted Fiberbow, who said my bolts were made from 11S aluminium. This is the same as the 2011 alloy, and is normally heat treated at 160 C for 14 hours. As this is considerably hotter than I need to cure the paint, I can be confident that if the bolts ever fail, it won’t be because of a 10 minute treatment at 120C as it’s insufficient to change the microstructure of the alloy, and hence its strength and resistance to fracture.

Painting

Before painting, I marked exactly where each bolt lined up with riser. I washed everything with dish soap and water, then dipped it in isopropyl alcohol and let it dry. I made sure not to touch anything while painting to avoid transferring grease/dirt from my hands.

I attached each bolt to a cordless drill with a wooden pencil so that I could rotate it while holding the brush in place. This gave a better result as it reduced the visibility of brush strokes. I have each bolt two coats. The paint isn’t fully opaque, which gives the colour a nice depth.

Here are the tiller bolts fresh out of the oven.

I also did some touch ups on my blue Beiter pressure button, which I’m not terribly careful with. The paint doesn’t hide the scratches completely, but they’re definitely less noticeable from a distance.