24th June, 2019

Workshop Guide: To Alloy Polishing

Words and photos by Steve Cooper

When new, engine covers of Japanese machines were either polished and lacquered or painted. Add time, stone chips and ultraviolet light and the coating breaks down. Add in some road salt and the alloy starts to fester away gradually delaminating more of the protective coating to a point where the casting grows a furry grey overcoat.

Painted cases can be cleaned back, filled where necessary and then repainted; all in all, a straight forward job. Polished case rectification takes a little more effort. It’s a messy job but someone has got to do it; so that’ll be us then presumably. There are several DIY polishing solutions out on the market and it pays to have a look at what’s available.


The kit should do pretty much all we need for an average bike and comes with the all-important dusk mask etc. This is a messy and noisy job with bits of mop, alloy and buffing soap getting flung about so a dusk mask, strong gloves and safety specs are the absolute minimum needed to work safely; I also add in a disposable boiler suit and earplugs. Make sure absolutely nothing like sleeves, cuffs, belt etc can get drawn into the polishing mop. To be able to polish effectively the alloy needs to be free of grit, grease, dirt and protective lacquer. The first three can be easily removed with a brush, some bike cleaner and hot water. The lacquer needs more aggressive techniques and paint stripper is normally very effective in removing this. It’s also possible to blast lacquers and paints off but bear in mind the surface may be slightly rough; it’s OK for paint primer but will take some extra buffing to get back to a notionally smooth surface.

Here I’m using gelled aerosol type stripper which clings to vertical surfaces while it does its job.

Alloy with gelled aerosol stripper sitting on newspaper

Another option is to use one of the more aggressive abrasive kits with either satine mops or abrasive polishes. However, remember that whilst these abrasive processes will remove overt damage they are also marking undamaged alloy. Because we are dealing with abrasive there will be a need to remove the fine scratches imparted on the alloy by the process. 

Alloy and polishing equipment sitting on workbench

With the alloy clear of dirt and coatings we can get a good look at the metal underneath and make a reasoned assumption as to how good or bad it is and what steps we need to take. If the alloy is just dull and unmarked it’s viable to start with the coarsest mop (a sisal one in this case) and some polishing soap of the corresponding grade. However, if the alloy is badly marked, scratched or pitted it may be necessary to bring in some strong arm tactics. Huge pockmarks or holes are better treated with one of the alloy welding kits advertised within CMM; with a bit of practice it should be possible to effect good repairs.

Yamaha side case

This Yamaha XS650 side case had suffered from damp storage and was quite seriously corroded but it has been brought back to a uniform condition with emery cloth, DIY flexible sanding blocks and decreasing grades of wet-and-dry paper.

Man using a second cut file on Yamaha side case

I’m also using a second cut file to carefully shave off any big dings while still preserving the original profiled and then redressing as above.  By using progressively finer abrasives (whether by hand or on a mop) it should, with time, be possible to get surface that will readily polish up to an acceptable level. Finally, unless you have a workshop area you can dedicate to polishing it’s probably best to do it outside; it’s inevitably a messy process and you don’t want the debris all over your bikes or work area.


Selecting the appropriate tail or pig for the direction of rotation of our drill or grinder the sisal mop is screwed on. With the mop running we can add some of the buffing soap designed for that specific mop; a quick one or two second application is normally sufficient.

Alloy sitting on workbench and mop.

Now we can finally bring the alloy (in this case a rare carb cover) into contact with the mop and by applying pressure the dull surface can be removed. More soap is added when the buffing tails off. It’s important to move the work piece around to prevent hollowing; also, the angle of attack should be varied so that the alloy is not all polished in the same direction.

If this happens you will see parallel tram lines along the surface that will show up on the finished item. The sisal mop is used until the work piece is uniformly buffed and further polishing doesn’t enhance the sheen.

Close up of alloy on workbench

Next, we move onto the second, softer, stitched, mop which will remove the coarser mark of the sisal one. Again using the appropriate buffing soap we can repeat the process using similar techniques. The second cut mop is often the one that does a lot of the work but at a slower overall rate than the coarser sisal version. There’s sometimes a tendency to think the effects are tardy and add extra soap. If this happens it’s possible to end up with extremely tenacious, big black, smeared, scuds of soap and metal particles on the polished surface.

Alloy and brick sitting on workbench

If this happens the excess soap can be removed by carefully running a clean wire brush over the rotating surface of the mop. Don’t be tempted to use abrasives or an old brick to do this as any hard particulates picked up on the mop will then damage the article being polished.

Man using wire brush to clean mop

Wire brushes are generally unlikely to leave bristles in the mop face. Finally we’re onto the soft calico mop and removing the polishing marks left by the stitched mop. Time taken here to cover every facet of the substrate will be rewarded with a gleam that will satisfy most of us.

Man polishing metal

Then we need rub the surface over with some Vienna lime to remove any residual polishing soap, grease or finger marks and the job’s a good’un as they say. All that’s left to do is possibly give the readily oxidisable surface coating of protective wax polish and it’s time to move onto the next job. Sharp eyed readers will note I’ve not polished out the marks in the top face of the carb cover. As the intended bike has a patina of wear I’ve elected not to go over the top but rather carry out a sympathetic refurbishment. 

If there’s a fair amount of polishing to do it makes sense to do it outside and have sufficient mops and buffing soap on hand. Always allow more time than you think the job will take if you’re new to this and aim to finish a certain number parts with the final calico mop rather than do all the roughing out in one hit. Firstly you can then do something else next time rather than get terminally bored with a bike’s worth of polishing. Secondly you get to see some genuine results for one days work rather than moving from dull grey to slightly brighter grey.

alloy and mop sitting on workbench

The results speak for themselves here.  Polishing is unarguably a messy job but it’s also disproportionately satisfying. An hour or so spent in the workshop with some grotty old bits of festering alloy quickly produces something to be proud of. Polishing can provide a therapeutic break in a restoration project; when you’re waiting for parts, the penetrating oil still hasn’t done its job or it’s too cold to spray that side panel there’s normally something that needs a session on the mops. Crank up the polisher and, for once, bask in your own reflected glory.


As with all things the amount of energy expended on a job dictates how good the results will be; it’s not all about how hard you polish though. The annoying phrase…don’t work harder, work smarter….is very appropriate when it comes to polishing.

Within reason a larger power source will give better results, you’ll need to expend less time/effort/energy and there’s less likelihood of damaging the shiny bits. Bench or post mounted grinder/polishers generally have the guts to do the job but you need to match the mop size to the motor’s capabilities.

As a rough rule of thumb a 370-500w electric motor should cope with a 6” mop. For 8” mops you’ll need a minimum of 500w and more if you can get it. On intricate castings or small parts it’s viable to use a cordless or normal powered drill and even on some large castings the easy access afforded by a drill makes the job simpler.

The bottom line here is that if the mop slows down substantially you don’t have enough power to hand. Also consider the size of the work piece. Large objects may be heavy and it’s probably easier to polish something like a bike wheel on a bench with a 4” mop on a drill than it is to hold the wheel up to a bench mounted 6” mop.

It’s also worth investing in a kit that comes with the smaller dolly mops (1”, 1 ½” & 2”) as these can be a godsend. The larger mops cannot get into recesses or tight spaces but the smaller mops can get into the nooks and crannies. Don’t throw a dolly mop away until you’re absolutely sure it’s beyond use; it’s guaranteed that you will need a worn down 1” version the minute you bin it.


It’s very easy to get just a little too obsessive with polishing and lose sight of what you are trying to achieve. Most of our machines were never particularly well polished when new. The alloy castings were delivered en masse from the foundry experiencing a few nicks and dings along the way. After a quick clean they would have been linished on a belt sander to remove any casting flash and polished.

Following a vapour degrease to remove any polishing soap the alloy was sprayed with a clear lacquer and that was as good as it got. If you look critically at an NOS engine case you can see the nicks and dings; whether to take them out is a call only the owner can make. Japanese alloy tends to have fairly high zinc contents and this manifests itself on occasion as a grey/black line. Normally this zinc rich seam will run right through the casting and is therefore impossible to polish out. If you stand back a few feet from the job most of these marks cannot be seen so don’t beat yourself up for no good reason.


Having got the alloy back as you want it’s worth considering how you’ll retain the effect. It shouldn’t be necessary to use abrasive metal polishes on a regular basis so look for a good wax polish or one of the more benign metal polishes that are now available.

Metal polishes that contain preservatives and/or waxes normally bring back the lustre and leave a protective coat. As long as water and oxygen are excluded from the metal surface the underlying polishing work should remain largely unaffected.


It’s quite viable to polish the odd piece of alloy by hand using any one of the well-known metal polishes. The results are eminently acceptable and it’s a viable method for small articles or one offs. However, if you have a number of parts to shine, a set of mops makes a lot sense.



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The information contained in this blog post is based on sources that we believe are reliable and should be understood as general information only. It is not intended to be taken as advice with respect to any specific or individual situation and cannot be relied upon as such.