Aircraft PolishingAircraft Aluminum Polishing

 

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Polishing Tips

The following is a random collection of bits and pieces of information on polishing.

What can I put on the aluminum to protect it after I've polished it?

Can anodized aluminum be polished?

How can I tell if my aluminum is anodized?

Acid Washing

Attention to cleanliness is essential

Don't polish in the sun

Gray milky areas that won't polish out

Will repeated polishing remove the Alcad layer?

Keeping your aircraft / Airstream clean

Removing dried-on polish residue from around rivets, etc.

Sanding aluminum to remove serious scrapes and scratches

Pitted aluminum

Washing your aircraft / Airstream

Sunlight is the ultimate critic

Taking care of your Cyclo Polisher

My Cyclo polisher won't start

Mixing different brands of polish

What can I use to protect the aluminum after I've polished it? This is the most common question we get. Unfortunately no one has yet found a product that provides any significant degree of protection for any length of time without dulling the shine. Waxes are often tried and people have used everything from floor wax to Carnuba car wax. But the problem is that when it's time to re-polish, all of the wax must first be removed or it will contaminate the polish.

If your aircraft or trailer is stored outdoors you will get streaking and water spots as soon as it rains. The rain carries industrial pollutants and minerals and these form water spots that can be difficult to remove. And dirt washes off the top and runs down and streaks the sides. Unless you chamois-dry the aluminum every time it rains, streaking and water spots are inevitable.

There are some things you can do if you are fortunate enough to be able to store your aircraft or trailer inside. The most important thing is to keep the hanger or garage dry. We have rainy winters here in California and hangers tends to remain damp all winter. The sun warms the hanger during the day and the air picks up moisture from the floor. At night it cools down and the moisture condenses onto the airplane. By springtime the aircraft is covered with greasy film and pinpricks of corrosion everywhere. The solution is to provide some form of ventilation to pull in dry air. Wind driven roof-top ventilators are the ideal solution but just installing a ceiling fan to keep the air moving helps a lot.

You can also cover the aircraft or trailer with cotton sheets. This seems to keep the dampness away from the aluminum and has been very effective at preserving the shine. In a dry environment you can often go 2 years without re-polishing.

Acid Washing. Aluminum can be acid washed to remove chemical contaminants and oxidation from the surface of the metal. The acid used is phosphoric acid, which is a relatively mild acid. Acid washing has traditionally been used to clean and condition aluminum before painting but it also will clean milky gray areas or other discolorations that polishing sometimes can't remove.

Aluminum is very porous and tends to absorb and hold discolorations from a variety of products such as automotive rubbing compounds and even some aluminum polishes. If the owner has tried a variety of polishes in one area, it may not be possible to remove all traces of these chemicals by compound polishing. Sometimes the only thing that will work is to acid wash the affected area.

The actual process is fairly simple. A diluted solution of the acid is applied by sponge or brush to the affected area. The surface is kept wet for 3 to 5 minutes until the water sheets (stops beading) and the aluminum has turned to a uniform smooth milky finish. The acid is then hosed off. The big problem is controlling where the acid goes and making sure no drips or drops get on to adjacent areas. If the acid dries on it forms a crusty film that is very difficult to remove.

The result is a milky white finish that polishes off very easily with the Cyclo polisher and Nuvite S. Acid washing will remove chemical contaminants and light oxidation but it is not a substitute for compound polishing. It won't touch the heavy layer of oxidation typically found on well-weathered aluminum. It is strictly a surface cleaner.

One brand used in the aircraft industry is called Alumiprep 33. Click here to read the manufacturer's data sheet (Adobe .pdf file).

There are other acid cleaners that are much more aggressive than the phosphoric acid type. They typically contain hydrofluoric acid or sometimes even sulfuric acid. These are dangerous chemicals that should only be used by professionals.

Can anodized aluminum be polished?The short answer is no. Anodizing is a chemical conversion process that leaves an extremely hard finish on the aluminum. It provides excellent corrosion resistance but it is virtually impossible to remove. Polishing with our most aggressive polish will not remove it. Small parts can be dipped in an acid solution at a metal plating shop to remove the anodizing but that is not practical for large items such as travel trailers. The dull gray finish will always be there. The best you can do is to scrub it clean and hope it looks a little better.

It is true that oven cleaners such as Easy Off will remove the anodizing but again that is practical only for small items with thin anodizing. I tried stripping the anodizing off the front forks of a motorcycle and it took four applications to get through the anodizing.

How can I tell if my aluminum is anodized? Many vintage trailers were made from anodized sheet aluminum. If you have a trailer that you think might be anodized, take off a tail light, name plate, or similar item that has protected the aluminum underneath from the weather. If the aluminum under the tail light is dull gray with a slight roughness to it, it is probably anodized. As an additional check take any metal polish and hand polish a small area under the tail light. If the aluminum is anodized it will remain dull gray. If the aluminum is not anodized the aluminum will turn bright.

With small items such as motorcycle brake calipers it is difficult to distinguish between something that has been powder coated and something that has been anodized. Anodizing can be done in many different colors so color is not a direct indication. I don't know of any way of telling for sure without damaging the coating.

Attention to cleanliness is essential.Any particle left on the surface or on the cloth will scratch. A grain of sand under the polishing heads will do an immense amount of damage in a few seconds.

Clean the area you are going to polish just before you begin.  Keep all your equipment clean. Never set your polisher or cloth on the floor. Keep all your equipment on a moveable cart or table.

Don't polish in the sun. Do your polishing inside if at all possible. If you have to polish outside, polish in the cool of the morning or on a cloudy day. If you polish in the sun the polish will harden and not work well.  If the day is hot and dry you may want to lightly mist the area to be polished to cool it.

Gray milky areas that won't polish out. One common problem with polished aircraft is gray areas that are visible in the sunlight that seem to be impossible to remove with any amount of polishing. The milky gray is caused by polish that gets worked into the grain of the metal. Until now nothing would remove it except acid washing. But we've had good luck getting rid of the gray with Nuvite G-6.The F-7 medium coarse may also remove the gray; we just haven't tried it.

We've discovered that a good many products will work their way into the grain of the metal and leave a residual discoloration. Rubbing compounds and polishes intended for automotive use are common culprits but some aircraft aluminum polishes seem to do it too over time. Aircraft polished regularly with one brand in particular that was very popular a few years ago seemed to end up with more milky areas every year. Eventually acid washing was required.

Nuvite S, the finish grade, seems to be perform much better in this respect. And the coarser grades seem to be good at getting rid of some of the gray. But sometimes only acid washing will remove gray.

Will repeated polishing remove the Alcad coating? Not according to Boeing. As a quality control measure Boeing polishes all of their exterior skin sections after forming using robotic polishers and Nuvite F-7.  Apparently defects in the skin are easier to spot when polished. They once polished a section 370 times and the surface cladding (Alcad) was still there. Boeing says that polishing does not remove metal, it just "turns it over."

Of course if the surface is badly corroded or has been sanded, the Alcad will be gone. But it will polish up nicely anyway.

Keeping your aircraft / Airstream clean.  If you venture out of the garage / hanger with your pride and joy it's going to get dirty. Here's how to clean it without dulling the shine.

Nu-Image from Nuvite is a cleaner/debugger that will does an excellent job of cleaning without affecting the shine. We offer it for sale on the Nuvite Aluminum Polish page.

Glass Plus is pretty effective in removing bugs and light stains and it doesn't dull the surface.

Auto spray detailers are also good for removing bugs and may provide a light protective coating.

Never use cleaners with ammoniasuch as Formula 409 or Windex. The ammonia actually etches the aluminum.

Pledge furniture polish will clean a really oily belly stain on your airplane. It will dull the polish some but it does a great job of removing the oil.

Use a chamois if your aircraft/Airstream got wet in the rain. If you just let it air dry there will be water spots. Then use one of the above mentioned products to remove the water spots. They can be tough to get off.

Washing your aircraft / Airstream.  Some people recommend washing with a mild detergent after you've completed the first round of polishing with one or the coarser grades of Nuvite but before the finish polish. We haven't tried it but it might be a good idea. But this is will not remove dried-on residual polish. Residual polish must be remove the same day it is applied. Once it dries on it is very difficult to remove, even with solvent.

But no one washes with detergent after the final polish. If you've got a lot of road dirt on your Airstream, I'd recommend washing gently with water  to get rid of the gritty stuff, chamois dry, and then clean with one of the products mentioned above.

Removing dried-on polish residue from around rivets, etc.

If the residue has only been there a day or two, it can usually be removed by applying a solvent and letting it soak in a bit and then wiping with a microfiber towel. We prefer using Mineral Spirits (paint thinner) as the solvent because it evaporates slowly and has fewer noxious fumes than solvents like lacquer thinner.

If the residue has been there weeks or months we recommend first trying the Cyclo polisher equipped with our 4 inch flat wool pads (#56-430, on the Compounding Polisher page). This pad along with Nuvite F7, used directly on the aluminum, will do a remarkable job of removing all but the most stubborn residue.

But if the residue has been there for years it can present a real challenge. Repeated soakings with Mineral Spirits followed by light brushing with a toothbrush will eventually remove the residue, but the toothbrush will leave fine scratches that must be polished out.

Some have reported that dabbing paint remover on the residue will loosen and remove it. This probably works but we haven't had a chance to try it.

Once you've got the residue removed, follow the instructions in the Application Guides "Using the Compounding Polisher" and "Using the Cyclo Polisher" to keep the black rings-around-the-rivets from reappearing.

Sanding aluminum to remove serious scrapes and scratches

Compound polishing will blend the edges of scrapes and scratches and tends to make the defect disappear because the sunlight no longer catches the edge of the defect. But really serious defects can be sanded out. You can sand lightly to just smooth out the defect in the same manner as the compounding polisher does. Or you can sand aggressively to remove enough metal to make the defect disappear.

Here's the general procedure: Start with whatever grit will remove the defect reasonably quickly. This might be as coarse as 220 grit sand paper. Then sand with progressively finer grits and finally use the compounding polisher to remove the last sanding marks. With soft alloys, generally doing the final sanding with 800 grit will be sufficient. The compounding polisher and Nuvite F7 will remove the 800 grit sanding scratches easily. With hard alloys, the final sanding should be much finer, such as 1200 grit, because the compounding polisher will not be able to remove coarser sanding marks.

A word about sandpaper: Always use aluminum oxide sandpaper. It is generally used dry. It is the only type of sandpaper that is approved for use on aircraft by the FAA. The black silicon carbide wet-or-dry should not be used because the carbide particles sometimes get imbedded in the aluminum and leave a slight gray tint that can only be removed by acid washing. And never use steel wool because iron particles will be imbedded in the aluminum and serious corrosion problems may develop.

Pitted aluminum

Salt, in the form of road salt or marine atmospheres, is the most common source of pitting of aluminum. Salt reacts readily with the aluminum. The first sign of pitting is usually a small pin-head sized bump on the surface of the aluminum. The entire surface of the aluminum will likely be covered with these pin-head sized bumps.

This bump contains the products of corrosion of the chemical reaction between the salt and the aluminum. Underneath the bump is a pit that previously held the aluminum that has now been converted into corrosion products.   Left to grow the bumps will become very visible bumps the diameter of a pencil eraser. Eventually the entire surface of the aluminum will be covered with coating of corrosion products. The base metal underneath the layer of corrosion will be badly pitted.

When the pits are small and shallow they can be usually be easily polished out. But left to grow, the pits become deep with steep sides. These pits cannot be cleaned out by polishing and will trap the polish and turn black.

What can be done polish pitted aluminum? The products of corrosion can sometimes be removed by compound polishing with one of the aggressive grades of Nuvite. But if the aluminum feels like sandpaper, then it is better to sand off the corrosion. But that still leaves the pits. The pits might be removed by very aggressive sanding but sometimes they are just too deep to be removed.

The pits will trap the polish and turn black as soon as polishing is attempted. The only solution is to do a solvent wipe immediately after polishing in an attempt to lift the polish out of the pits. If the polish is allowed to dry in the pits, it will be likely be there forever. Pits are just difficult.

Sunlight is the ultimate critic.  Every polish job looks great in the hanger under artificial light. But bright sunlight reveals every flaw and shows the gray splotches that seem to plague everyone.

Here's the usual scenario. You spend a couple of hard days polishing your airplane or Airstream. Then, tired and dirty but feeling like you've really accomplished something, you wheel it out into the sun to show it off. You're sure to have a major disappointment. It will probably look awful, at least compared to the way it looked inside a few minutes before. Relax, it happens to everyone.

Taking care of the Cyclo Polisher.  

Overheating:Give the polisher a break when it the motor housing gets too hot to hold your hand on comfortably. The Cyclo polisher is an incredibly rugged machine but it gets hot if used continuously. I've never known one to break (except for a bad switch) but if you should burn it out, repair parts are available from the factory.

Lint Accumulation: Lint from your polishing cloth can accumulate inside the motor housing. Terry cloth towels are the worst offender because they shed into long fibers that clog the air passages in the motor and act like an insulating blanket that keeps heat in. 

Lint isn't much of a problem if you polish with sweatshirt material. Sweatshirt material seems to shred into smaller particles that pass right through the motor.

If you have a source of compressed air it's a good idea to blow the lint out once a day by directing a jet of air into the top of the motor.

For serious accumulations you may have to disassemble the motor housing. First removing the brushes at the top of the motor housing (observe which way they go back in). Then remove the two screws on the bottom of the housing and the two screws at the end of the handle. Use a dental pick to remove the lint from behind the motor coils.

My Cyclo polisher won't start.  Quite often for no apparent reason your polisher may refuse to start. Apparently lint gets under the motor brushes or the brushes just land on a dead spot. With the switch off, grab the polishing heads and rotate them by hand. This will spin the motor and usually remove whatever has gotten under the brushes.

Mixing brands of polish. If you're experimenting with different brands of polish, do so carefully. Switching polishes and re-polishing the same area will sometimes leave a brownish discoloration. Apparently  the chemicals in the different brands will sometimes interact.

 
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