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A. I hope to be corrected, Domenick, because I don't think it's possible to reliably remove silicone contamination from aluminum parts. In fact, I'm not so sure you can reliably do it even on steel parts where much more aggressive cleaning is possible. The rule is that parts simply must not ever get contaminated with silicone. Good luck.

Ted Mooney   Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


A. The best way to eliminate silicone contamination is to bar the use of silicone oils in any processes prior to the painting operation. Once silicone gets on the surface of the parts, it is nearly impossible to completely remove it.

The following are some potential sources of silicone: mold release agents, defoamers used in rolling or forming oils, coolants, cleaners, water displacing lubricants like WD-40 [linked by editor to product info at Amazon], hard surface cleaners that claim to leave surfaces "shiny", sealants and caulking materials, personal care products like skin creams, hair conditions, and lotions and many other common products.

When attempting to clean silicone oil off the surface of parts, it is very common for it to redeposit back onto the parts that were originally affected or onto other parts that entered the process free from silicone.

Silicone will cause paint defects at the parts per million level making it nearly impossible to detect at low levels.

Roy Nuss
Trevose, Pennsylvania, USA


A. No recommendation but found this:

GenSolveTM High-Performance Stripping Agents remove tenacious polymers from metal [...] substrates. They are effective at ambient temperatures with [...] coatings made of silicone [...]

Good luck,

Jim Wood
- Concord, California


A. I'm trying to clean some silicone myself and found out that sodium hydroxide or another strong base can de-polymerize the silicone back to cyclics and would probably work okay on aluminum (not sure about plastics yet). Also I hear Hexane will do the trick (probably better for metals) if you rinse it a few times.

Rob Ward
- Oakland, California, USA

September 20, 2012

Q. Hello Sirs,

We have a problem to clean some aluminum parts. We tried degreasers, alkaline cleaners, acetone, and didn't work. We notice the contamination in the FPI inspection due to the penetrant break in the surface. Do you think silicone could be the cause?

We use coolants to machine the parts, we have been cleaning the parts for the parts 2 years with the same process and suddenly the problem started last week. We changed all the cleaning line chemicals and also cleaned the tanks.

Is there a way to test if silicone its present on the parts or in the coolant of the machines?

Yohands Rey
- Chihuahua, Mexico

September 28, 2012

A. Hi Yohands,

Silicones can certainly be detected. If the concentration is high enough then something like FT-IR will be a good tool, especially for any of the cutting fluids or coolants, otherwise it may need to be more specialised such as SEM with SIMS attachment.

What makes you suspect silicones, other than water break failure?

Cleaning of silicones is particularly difficult, but degreasing with standard halogenated degreasers followed by a good alkaline clean is usually sufficient. If you are finding that the film is still tenacious then it will come down to manual cleaning, this may involve some form of abrasive cleaning. If this is the case and the process is carried out prior to penetrant inspection you will need to carry out a pre-penetrant etch to remove smeared material.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

October 1, 2012

Q. Hi Terry,

In reality we don't know what its causing the water break, but we tried a lot of things to clean the parts and we only succeeded with Zep Dyna 680-T2 degreaser solvent then acetone and finally the clean line process.

Until now the problem is controlled with this cleaning but we want to find the root cause, so we can eliminate it and go back to our normal cleaning process.

The SEM analysis could be performed in liquids or do you mean on the surface of the part?

What else do you think could be the cause of the contamination or the condition in the surface of the parts, this only happens in the machined areas.


Yohands Rey [returning]
- Chihuahua, Mexico

October 4, 2012

A. Silicones are so difficult to remove that it is absolutely imperative that they are never allowed near any plating, painting or soldering process.
I would suggest that you should have a company policy banning them and make sure that your buying department understand the importance of this. Next you need to do a thorough inventory of the maintenance department and remove all those spray on wonder 'fix everything' lubricants they love. Hand creams are another common source. The ladies love them, particularly the ones they get as christmas presents! Health and safety departments are very good at handing out free samples of "skin care products" - they must get the message too.
There are no reliable cleaners, whatever the salesman tells you.
Just make sure that everyone understands the threat to the business (and their jobs!)

PS Oddly enough, I don't think that WD40 has silicones but you should check

geoff smith
Geoff Smith
Hampshire, England

October 8, 2012

A. Hi Yohands,

Geoff's advice is sound, but knowing the sort of business Aerospace is it isn't always possible to avoid silicones or silicone based lubricants.

SEM analysis would be done on the surface of the contaminated part, not any of the solutions. It should be reasonably easy to pick up the Silicon peak with this analysis technique if it is present.

I get the feeling you are going to end up having to clean these parts very manually, that is swab degreasing with solvent and possibly scrub cleaning with a non-silicated cleaner.

As for trying to find the root cause of your problem you are going to have to look at the types of parts that are being processed, are they sheet, wrought or cast? What machining operations do they go through? What coolants/lubricants are used in the manufacturing process? Are these materials potential sources of contamination? Is someone using one of the "magic spray" cans people seem to have hidden away all over the place that are perceived to help with manufacturing?

At the end of the day you can't beat "walking the process", that way you can follow the parts from receipt to packing, will be able to see what operations are involved and what chemicals are utilised.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

May 24, 2013

Q. We're preparing to repaint an aircraft. Swab tests show there is silicone on the surface of the old paint. Will chemical stripping of the old paint contain the silicone within the waste stream and keep it from contaminating the hangar?

Jeff Wilterdink
US Air Force - Tinker AFB, Oklahoma, USA

May 29, 2013

A. Silicon "on the surface" of the existing paint could be just a polish which was applied. I'd try scrubbing with something like Ajax and retesting a sample area to see if that will remove the silicon. Steam cleaning with a detergent might work too.

jeffrey holmes
Jeffrey Holmes, CEF
- Spartanburg, South Carolina

June 3, 2013

Q. Hello Jeffrey Holmes. It's good to hear from someone. You're correct in that the aircraft has been waxed to maintain appearance. I might need to clarify that we're not planning to strip the aircraft for the sake of removing the silicone. Rather, our paint vendor has said they are unwilling to receive our aircraft into their facility as long as there's silicone on it because they don't want to "contaminate" their facility with it. Consequently, we're currently making extraordinary plans to hand wipe the aircraft with a cleaner prior to sending it to the painter. Since they're going to strip it there (not just sand it) prior to painting, I'm currently in disbelief that the chemical strippers wouldn't adequately handle the silicone and keep it from contaminating their hangar. I want to see if others think that's necessary before we pay an exorbitant bill in additional labor hours for the surface cleaning. Additionally, does anyone feel that the water-break-free test for a clean surface is not adequate when checking for silicone?

Jeff Wilterdink [returning]
- Tinker AFB, Oklahoma, USA

June 13, 2013

A. I think worrying about the silicon residue from a waxing is unwarranted, however it's their hanger, so their rules.

Steam cleaning with detergent shouldn't be very labor intensive.

Jeffrey Holmes, CEF
- Spartanburg, South Carolina

June 13, 2013

Q. Hello Jeff Wilterdink
what kind of swab test did you use?
I've been swabbing and doing FTIR, but wondered if you had a wet chemistry test you were using to test for surface silicones - I'd like to skip the lab time in my testing.

Steve Kuethe
- Centennial, Colorado, United States of America

June 14, 2013

A. Hi Steve
The reason we don't want silicone is that it will not wet. Therefore the easiest test must be a simple water break check.
No swabbing a small area and hoping the rest is OK. Just throw a bucket of water over the aircraft or park it in the rain. You will soon see any residual silicone - and where it is so that you don't have to redo the whole thing if you find any.

Geoff Smith
Hampshire, England

June 15, 2013

thumbsup2Hi, Geoff. What was the specification number again for throwing a bucket of water over an airplane?  :-)


Ted Mooney,   Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

June 14, 2013

A. Lab swab testing is by hexane swabs.

Rick Norwood
- Oklahoma City, Oklahoma

April 14, 2014

Q. I am trying to find a DIY swab test to audit our product to ensure no silicone surface contamination exists. Does anyone know where to get a swab test from? I have seen many discussions about about swab tests but cannot find any for sale.

Thanks for your Help!

Curt Young
- Mansfield Ohio USA

December 17, 2014

A. I have been using two products by RPM Technology to completely remove silicone and silicone residue from metals and fiberglass. I use the Digisil to remove the larger bits left behind from mechanical removal. i put it on thick, cover it in plastic, and let it work over night. after a good cleaning and wash, i spray and scrub with the Di-Aqua, this removes the layer of slimy residue left behind. I am not affiliated with this company and I do not receive any compensation of any kind, it's just a great product that works for me, and I thought I'd pass on my $.02.


luke brannigan
boat varnish and paint - portland, maine, usa

August 21, 2015

A. Hexane is the only cleaning agent that will come close to displacing silicone. The trick is to heat RO water to 120 degrees, mix in the dry Hexane and use it with clean wipers. Dispose of the wiper after one use and repeat.

P.S. Evaporative solids testing will not detect silicone. The presence of silicon can be determined relatively rapidly by elemental analysis, e.g., atomic adsorption or inductively coupled plasma (ICP). To localize the contaminant, a two dimensional technique such as scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDX) might be used. The SEM provides a characteristic visual observation of a very small area (e.g. a coating defect or a particle). EDX elemental analysis of that area identifying carbon, oxygen, and silicon can be used as evidence that some sort of silicone is the culprit.

Brad Hawkins
- Arab, Alabama USA

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