Re-Painting the Hub Carrier

This post is less about starting a new job or finishing old tasks, but more about fixing a painting mistake on the rear hub carrier. Luckily Mark had already spotted this error and explained a better way of painting cast aluminum, which in hindsight I believe would have resolved this problem. The one downside to this was that due to my slow posting on this blog, Mark’s response to my original painting post came just after I had spent another weekend working through the steps below to fix the issues. Still, I did explain why I had observed the painting issues, which put my mind at rest. Importantly as this post will show, this problem was easily resolved with filling and re-painting.

Flaky Paint

Due to Christmas holidays, several weeks (maybe a month or so) went by between me painting the hub carries and then looking at them again. When I did, I noticed that the paint, where the splash shield fastens, was starting to peal. More concerning still was that this issue was visible on both hub carriers. It only seemed to be in this small region, which I guess is because it’s these areas which had the greatest amount of corrosion, and I didn’t manage to remove it all. 

Flaky paint on the rear hub carrier. 

Flaky paint initially sanded off.

Filling down and re-painting

A quick rub with sandpaper removed the large flaky parts. Although all the loose paint had been removed, I suspected that the problem would keep coming back unless I followed a more aggressive sanding approach. Doubting that I had not removed all of the corrosion, I, therefore, began filling down the entire flat area to remove as much of the corrosion as I could. Plenty of partially corroded aluminum quickly came off, further highlighting the error in my earlier ways. Lots of filling later, the surface was looking a lot better, although still with some deeply corroded recesses. Whilst I was able to get it looking a lot better than the image below, I was not able to remove all of the deep regions. 

Partially filled down hub carrier.

To avoid a repeat of the flaky paint, I then decided to seal this surface using POR 15. After carefully masking up the freshly installed bearings, I applied two coats of POR15 to the affected area. 

Preventative coating of POR15 on the affected area. 

After leaving this to dry, it was then time for a fresh coat of grey primer before a top coat of Arctic Silver. When spraying the primer, I used extra newspaper around the carrier to avoid getting too much overspray on the rest of the hub. 

Hub carrier, outside ready for first coat of primer. 

Hub carriers after the primer is applied.

After all this work is said and done, the final part looks as good as it did before. I don’t think you could tell that it has been partly sanded back and re-sprayed. 

Re-painted hub carrier.

The Cause

As I said at the start of this entry, Mark did explain the potential cause for why this had happened, all before he knew I had encountered a problem. The explanation was that cast aluminum is slightly porous, with containment and gasses trapped inside. To avoid this reacting with your paint, it is recommended that you place the part in an oven for 30 minutes to extract these contaminants/gasses before painting. It was this oven step, which I missed when I originally painted the carriers.

However, remember, I did put the painted carries into the oven, to allow them to expand when installing the races. I suspect this resulted in the same expansion and extraction gasses/containments from the aluminum, although this time they reacted with the paint which leads to the flaking. Although I learned of the correct approach after fixing the original problem, I’m hoping the POR15 will provide an adequate seal. I will keep a close eye on this area over the coming months and years, in case further action is required. 

Looking back, I am glad this happened, as I would never have learned about the need to degas cast aluminum parts before painting. I’m sure I will continue to make mistakes and go back and fix them! So, if anyone does spot other errors in my work, please leave a comment below, im always happy to learn from my mistakes (It certainly happens enough).