M coupe Part 3: Polishing the Differential Cover

I'm still on the fence about whether or not this was worth it. I am also on the fence about whether or not I am finished with it... I need to remember though, that this piece sits under the car and will face the elements. The time spent making it look good was enjoyable. Time spent making it "perfect" may simply be wasted...

So, I also want to preface this by saying this was the worst looking M diff cover I have ever seen. I believe these are made by using a "sand casting" method in which the material is poured into a mould made of a special type of sand. Over time, the mould wears, leaving deeper and deeper casting marks. I only took one decent before picture, as seen below, but the cover was ...covered... with deep, ugly casting marks.

As soon as I touched the dremel to the piece, I basically knew I was in for an extremely tedious process. I think I took this picture as a "What have I done?" moment washed over me, but I'm glad I did - it really shows the difference in finish from where it started, to the shine it reached in the end.

Before this project, I had never polished something like this before. Let me clarify - by "anything like this" I mean I had never polished anything metal before. Didn't I say in a previous post that I didn't want to bite off more than I can chew? So why the hell did I try to learn to polish on something with as many complicated curves and inside corners as this piece?

Anyways, at first I started sanding by hand. I took the bigger casting marks out and then tried to figure something out for inside the fins. I realized pretty quickly I might be in trouble with these fins... how stupid would it look if the cover was shiny but then between the fins was a crappy, different looking finish? Sandpaper, even reinforced with a solid, flat piece of metal, wasn't cutting it.

I don't have any pictures (I took a timelapse video I'm working on editing) but bought a set of metal files and went to town. That got it... accetable between the fins. You can still see some sanding marks but it doesn't stand out notably against the rest of the cover.

I don't think I took as many pictures as I should have, I think the below pictures are after the first polishing step was done. By this stage, I had hand sanded it, filed between the fins, gone over it with rotary abrasive wheels and then used a dremel with a buffing bit and polish to polish it all.

Finally - I went over the cover with Mother's Magnesium and Aluminum polish. Rub in, buff off... so much black came out. Brought the shine right out.

The camera picks up minute sanding marks that aren't visible in person, but that's not to say it is a perfect finish. Far from it, but I think it will be good enough to be installed. I may take the one on the car now, and try my hand at polishing that one - applying what I've learned this time, to try to get a nicer result.

Before and after below, just for comparison sake

I can't wait to peek under the rear of my M coupe and see those fins shining back at me...


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