Topic: Polishing tips - discoloration and fins

[cshanek]

I began polishing my bike yesterday to take advantage of having the carbs and exhaust off. The left side started to polish up pretty well but the right side and area under the carbs has a lot of discoloration (see pics). Any advice on the best way to deal with this?

Also what is the best way to clean and polish the fins on the block? I have already pressure washed the bike and there is so much caked dust/gunk on them that I am seriously just considering using a wire brush or wire brush attachment. I used 600 grit sand paper on the left side, folded so that it could barely fit between fins, but that took a long time and only seemed to clean the edges.

[Pielz]

I was able to clean a bit better between the fins with a soft thin fiber disk that just barely fit between the fins, its green brillo material but on a drill attachment. Kinda slide it in there soaked with marvel mystery oil and use the drills power.

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[Kevin D]

Paint?

Cases, fins are painted
Side covers, head cover polished

Yes, you have to clean the crud off before paint.

[pjlogue]

The dark "stains" on the aluminum is probably smut.  It is caused by the different elements in the alloy of the Aluminum, most notably, copper.  The stain is hard to remove by polishing.  Wet sanding will remove it.  Most people will polish the engine covers and paint the case, cylinders and head.  Trying to polish between the fins of the cylinders, I think, would be an endless job and very frustrating.  Any tool you can find or make that will fit between the fins isn't going to be big enough to process any meaningful surface area quickly.  Think of trying to polish something the size of your gas tank using the tip of a pencil erasure.  It ain't going to be quick!

Your best option is to clean all grease off and prep the surface with fine wet/dry sandpaper or soda blast it before painting.  Soda blasting is very messy and you need to be absolutely sure all possible entry points into the engine are sealed tight.

-P.

[cshanek]

Thanks for all of the information and responses. I had read in the past about certain elements leaching to the surface to cause discoloration. Interesting.  I will give the brushes a go and do some reading on painting. I have a cb500 with a porcelain paint job on the head/block and it came out wonderful.  I would like to keep my 750 as close to stock as I can though.

[754]

I always thought it was the zinc in the alloy coming out..  if it were copper it would turn green.

[pjlogue]

Copper Oxide turns black.  The green patina on coper flashing  is from sulfides in the atmosphere but over time it turns very dark/black.  I had read it was from the copper in the alloy that made the dark smut on aluminum castings.  I tried to see if I could chemically remove the dark blotches when I restored my 750.  Sanding it out was the only realistic solution if the smut was deep. 

-P.

[cshanek]

I ended up just rolling with wires brushes and find sand paper for the fins. Wet sanded some of the discoloration out, and buffed and polished all of the chrome. I could do better but not bad for a first pass.