Polishing metal parts on your motorcycle
Whether restoring a modern classic, designing a custom bike or simply restoring a used bike, at some point or other you will inevitably need to polish aluminium, steel or chrome.
Which material should be polished?
With a polishing kit for your power drill (see Fig. 1), you can restore unsightly tarnished and dull aluminium parts to their original splendour, and sometimes even better than new. Similarly, shining covers or stainless steel screws create a striking contrast against a black-painted engine – and highly polished stainless steel parts can almost look as if they are chromed. If you're planning to have steel parts professionally chromed, polishing is the most time-consuming, and thus most expensive, part of the preparation work. So if you have a polishing kit and know how to use it, a little bit of elbow grease will make an amazing difference to the way your bike looks. The pleasure you get from the shining results of your work will make all your trouble worthwhile (see Fig. 2)!
To do this job properly, you first need do know what sort of metal the parts you want to polish are made of.
Engine cover, fork sliders, controls and levers, rear wheel swing arms and other add-ons made of aluminium can be identified by the fact that they are very lightweight, non-magnetic and often painted. Look at the parts very closely – silver paint on the engine block, or layers of clear lacquer on outer covers are often quite difficult to recognise. Over time, corrosion often creeps underneath the paint or lacquer – really NOT a good look! (see Fig. 3). Before you can polish these parts, you need to sand off the paint or lacquer and the corrosion. Corrosion often sits deeper than expected, but it must be removed without trace to prevent it recurring. Colour or silver anodising is also widely used to protect aluminium against the elements or to highlight a specific component. This coating is characterised by a metallic, slightly matt appearance (see Fig. 4). Over time, cleaning can make the anodised finish appear dull and threadbare – in which case, it's better to simply sand off what's left of it and polish the component.
Aluminium frames, swing arms
Always take particular care when polishing modern aluminium frames and swing arms, as removing too much material can weaken these parts. Never sand down welded joints! Many approval authorities in Germany require a special report for a polished frame – so before you start, it is a good idea to contact a vehicle inspector you can trust to find out whether any special approvals are needed. Moreover frames and swing arms, whatever material they are made of, must never be chromed.
Parts made of steel are distinguishable from aluminium by their heavier weight and because they are magnetic. They always require a protective coating to prevent corrosion. For this reason, chromium plating or bright nickel plating are an attractive alternative to painting. Chromed fittings in particular offer an attractive contrast to the paintwork, while larger chromed parts (such as the tank) just look plain cool.
However, since these electroplated coatings do not have any filling characteristics, the parts must be carefully polished before treatment. This is time-consuming and expensive – so by doing the work yourself, you save money and will not have to wait so long for your parts to come back. However, you need to do a perfect polishing job so that the galvanising shop only has to put the parts into the electroplating bath. Frames and swing-arms made of tubular steel must not be chromed, but they can be bright nickel plated. Before you start work, check that your vehicle inspection authority will permit any changes you make – better safe than sorry! Parts being nickel-plated also need to be very thoroughly polished – welded joints must not be sanded down!
Stainless steel is easy to distinguish from ordinary steel, as it is only slightly magnetic, if at all, and it does not normally rust. Polishing lends it a chrome like, high-quality finish (Fig. 5 – Stainless steel cap nuts, polished and unpolished). However, you need a lot of elbow grease for this task, as stainless steel is extremely hard. For particularly good results, use a special wax for stainless steel.
Sanding and polishing are both jobs that create plenty of dirt and dust. A lot of fluff comes off new polishing mops at first – this is normal. So it might be wise to work outside or to prepare a room so that the dust cannot cause any damage, i.e. by covering shelves with a plastic sheet from your local DIY store and clearing away anything that might be harmed by dust. Wear suitable work clothes (such as Rothewald overalls) and, most importantly, always wear a dust mask and protective glasses/goggles! For polishing you will need a workbench with vice (or screw clamps), a drill, a clamping device for the drill and a polishing kit and some brake cleaner – as well as some wet/dry sandpaper with assorted grits 80-600, and maybe an angle grinder with a grinding disk for the preparatory work.
Removing the paintwork
If you want to save yourself the time-consuming job of sanding to remove old paint and corrosion, you can always go to a professional sandblasting firm. As well as conventional sandblasting, there are now a whole range of more gentle processes, right through to dry-ice blasting. And don't forget: any engine parts that have been treated by sand/glass bead/walnut shell blasting must be very thoroughly washed before they are installed! It is crucial that no blast material is allowed to get into the engine. We do not recommend blasting an entire engine with conventional blast material – even if you do tape over all the delicate parts – only dry ice is suitable for this task.
If you only need to remove a layer of paint, this can also be done in a paint-stripping workshop, which strips the paint chemically or thermally. You can also strip individual and small parts yourself using a good professional gasket and paint remover (see Fig. 6). such as Nicro 941, which works very fast and produces very good results (always wear rubber gloves!). Paint strippers from the DIY store are not generally suitable for automotive paints.
If you don't want use the services of a paint-stripping/blasting firm, and you haven't got a suitable paint remover, you can also sand away the paint and rust. The quickest way to get good results is to use an angle grinder fitted with a flap wheel (see Fig. 7). However, this solution should only be used if there are quite deep scratches/grooves and heavy corrosion. Always wear a facemask when using an angle grinder.
Difficult to reach areas or small parts can be manually sanded down with wet/dry sandpaper or with a "Dremel" – wet sanding helps prevent dust formation. Choose the grit of your sandpaper carefully, depending on the surface you are treating – the coarser you begin, the longer it will take you to achieve the final finish – on the other hand, if you start too fine, it will take forever to remove deeper imperfections. Sensitive sealing surfaces or inner surfaces of the component should be protected before you start work by covering with masking tape (see Fig. 8)! To sand down the component, first clamp it in a vice or fix it to the bench with a screw clamp so that you can work safely. Use wood, cardboard or special clamping jaws to protect aluminium parts against the sharp edges of the vice or screw clamp. Take care not to weaken load-bearing parts by removing too much metal. Always work cross-wise to achieve an even surface, never sand in just one direction or for too long in one place. Especially when working with an angle grinder, take care not to make any hollows or grooves!
After coarse sanding to get rid of corrosion and paintwork, you need to fine-finish the surface with wet sandpaper to get it nice and smooth. Only a properly smoothed part will look good because a polished surface really shows up any imperfections. You need to take care on fluted parts to avoid creating "hollows" – the best way to do this is to use a sanding block (see Fig. 9). Finish off with very fine 320 grit wet sandpaper – or better still 600 grit. You should end up with a surface that is very slightly rough from the sandpaper, but smooth and even enough for you to now polish.
Put a polishing mop on the spindle and insert in your power drill. Start with the coarse sisal cutting mop. You can either fix the power drill to your workbench using a special clamp (see Fig. 10) or clamp the part to the table. To be on the safe side, cover the chuck with a thick layer of masking tape so that there is no risk of it damaging the part if there is any accidental contact. Always wear a facemask and protective goggles when polishing.
Press the pre-polishing wax against the rotating cutting mop until you see it change colour – which indicates that it has taken up enough wax to start work (see Fig. 11). This is easier if the wax is not too cold, ideally room temperature – place it near a radiator for a while if necessary. You'll find that the new mop will leave fluff behind at first, but that's normal.
Once the mop has taken up wax, you will only need to press it briefly against the wax to top up later. Polish at a medium to high speed (2500-3000 rpm), applying light pressure and with circular movements (see Fig. 12). To avoid making hollows, work outwards from scratched areas in a spiral. Clean with brake cleaner and check the result. If the surface looks even, switch to the medium (cotton) mop and the relevant wax for smooth polishing. Finally, clean the part again with brake cleaner and then switch to the finest cotton mop and the fine-polish paste for the bright finish.
Important: Only use your polishing set, comprising mops and wax, for one type of metal. For example, a mop used to polish stainless steel should never be re-used on aluminium – you would not get a bright finish.
Caring for polished surfaces
Unpainted aluminium parts can be cared for with aluminium polish, especially after riding in the rain. Water spots and "rust bloom" are best treated by waxing, or possibly wiping down with an oily cloth. Clear protective lacquers do not adhere well to the smooth surface – if you then get "corrosion worms", you need to remove the lacquer and re-polish.
Aluminium parts can also be chromed if you want to avoid this maintenance work. Expertly applied three-layer chromium plating (layers of copper/nickel chromium) works well even on thermally stressed engine covers – provided the surface is clean and free from pores. Aluminium that has pores or which was previously deeply discoloured is not suitable for chromium plating – there is too great a risk of the surface blistering fairly rapidly. The question of whether or not to chrome aluminium ultimately comes down to style – classic and old school motorbikes and machines with "pure technology“ styling often simply look better with the authentic warm tone of untreated aluminium. Polished stainless steel is very easy to take care of – any fine scratches, watermarks or discolourations (on exhaust systems) can be removed with stainless-steel polish from Autosol.
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These tips for DIY mechanics contain general recommendations that may not apply to all vehicles or all individual components. As local conditions may vary considerably, we are unable to guarantee the correctness of information in these tips for DIY mechanics.
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