Removing urethane efc100 fueltank coating

[StevenL757]

11 hours ago, mike_elliott said:

Walter Dodge of Kelly Aerospace (wdodge AT Kellyaerospace.com) was involved with the original STC and licensed it to Midwest M20 to apply on Mooney's. He may have some info that could be of help. I am curious, did it develop a leak, or are you just cleaning it up and getting some UL back? Kevin Surrell (RIP) was emphatic that it would never leak back in the day when he was doing this sealant.

@Hradec, I also strongly suggest you contact Paul Beck in Willmar, MN.  He’s a leader in the fuel tank repair and strip/reseal business who could likely give you a hand here.

https://www.weepnomorellc.com

Steve

[Raymond J1]

3 hours ago, Hradec said:

Vous avez appliqué le même stc à vos réservoirs avec l'uréthane?

In my wings the original PRC was repaired with its French equivalent of the brand " LJF " (the French Joint). I worked for a period at the manufacturer Dassault, assembling the Central plane of the Falcons series, I am used to the "PRC", so I keep this type of sealant for my tanks.

I removed this PRC with this type of cleaner which is suitable for PU and epoxy resins. The softening is not lightning, but it is enough for a gentle mechanical stripping with a brass wire brush, put at the end of a slowly rotating drill hose. The important thing is not to mark the aluminum, so doing it gently, in a thin layer and in several passes suited me very well. I am of the opinion of mooniac 15u, it is necessary to be careful with liquid cleaners that interfere in the joints of sheet metal and can still act in time because never neutralized. Even for the stripping of the exterior paint I proceeded with gel remover, in small areas there were many washes with water.

For the wing, I proceeded like Mooney by putting the piece vertically on a support made for the occasion.

 

Edited December 18, 2020 by Raymond J

[Hradec]

3 hours ago, StevenL757 said:

@Hradec, I also strongly suggest you contact Paul Beck in Willmar, MN.  He’s a leader in the fuel tank repair and strip/reseal business who could likely give you a hand here.

https://www.weepnomorellc.com

Steve

I contacted paul, he was really great. My call sent him down memory lane

There was a 2 year period around 2000 where about 100 aircraft had their tanks resealed with the efc100 urethane stc. 

His process wont budge the material

The urethrane is great until you need to remove it, then you are screwed!

I called FFC. Apparently the military had its 707 aircraft use this system, it was great until it came time for removal during heavy inspection. They went back to the old school prc polysulfied sealant.

Apparently the contractors had to follow strict environmental  guidelines. The removed it with the combination of 15000 psi power washer and mechanically removing it with scrapers,  it was a horrible job

[Hradec]

14 hours ago, mike_elliott said:

Walter Dodge of Kelly Aerospace (wdodge AT Kellyaerospace.com) was involved with the original STC and licensed it to Midwest M20 to apply on Mooney's. He may have some info that could be of help. I am curious, did it develop a leak, or are you just cleaning it up and getting some UL back? Kevin Surrell (RIP) was emphatic that it would never leak back in the day when he was doing this sealant.

I ended up speaking with Mr Dodge. Has a guy that was working on this stuff, waiting for a response from his guy on what he used to remove the urethane. Sounds like the urethane was a great idea, very durable, but you were screwed if you ever had to remove it.

 

[Gary0747]

10 hours ago, mooniac15u said:

I'm more of an organic/medicinal chemist so I haven't done any polymer chemistry in a long time but I seem to recall that crosslinked polyurethanes are extremely solvent stable.  You probably won't be able to get it to actually dissolve.  You may be able to get it to swell and separate from the metal with something like methylene chloride.

 

3 hours ago, mooniac15u said:

Methylene chloride is fairly volatile and doesn't leave a residue.  It has a lower boiling point than acetone.  There is little reason to be concerned about long term effects on aluminum.  Of course you are unlikely to be able to buy any unless you work in a lab so it's probably a moot point either way.

Sounds like all your experience is working with Pyrex working in the lab?  With 32 years experience running plants in the real world that made this and other chlorinated organic products I recommend not using them on aluminum!

[carusoam]

Gary are you a chemist as well?

Whenever MS has basic chemistry questions, we tend to lean on 15U for his experience...

It is great to know everyone’s back ground... we don’t have enough chemists around here...

We are just finding out how much experience JR has... and where some of it came from...  (how cool is that?)

When they laid out the details of PU and how it branches and cross links differently in different applications... and some details about safety to go with the solvents...

I didn’t find it necessary to shoot at them for the things they may have left out...

Welcome to the weekend rules at MS... 

Best regards,

-a-

 

[Gary0747]

4 hours ago, carusoam said:

Gary are you a chemist as well?

I am a Chemical Engineer with an advanced degree  having worked 32 years for Dow Chemical which was  the largest manufacturer of chlorinated organic chemicals in the world.   I know from real life experiences how incompatible these materials are with aluminum. I have to speak up when somebody acting as an expert “corrects”me and makes a bad recommendation that will likely cause harm. Methylene chloride will get into the cracks and joints and over time decompose to minute amounts of hydrochloric acid which will damage the passive protective oxide layer on the aluminum causing exfolation of the aluminum in the crack or joint. Raymond J eluded to caution when talking about cleaning preparation for tank lining above.   I have also felt that some people painting aircraft have not been aware of the hazard here and have used chlorinated solvents to strip the paint aircraft. This type of corrosion will not show up immediately but years later the results can be exfolation damage showing up in the cracks and crevises especially in the overlapping joints and rivets. Yes these solvents are volitale but they can become trapped or mixed with grease, dirt or other materials so don’t just assume the evaporate cleanly like they might in a laboratory beaker.   Do we really understand all the causes of corrosion on our airplanes that we are scraping everyday?  This is just one.  

[carusoam]

58 minutes ago, Gary0747 said:

Do we really understand all the causes of corrosion on our airplanes that we are scraping everyday?  This is just one.  

Gary, it is great that you shared where you are coming from... thank you!

I think what our OP is battling may be similar to what you have described...

He is fighting some corrosion that isn’t a normal find for most Mooneys...  in area that is on the back side of the fuel tanks...

 

One thing we have learned around here... once corrosion starts, it can spread quickly...

Thanks again, and best regards,

-a-

[carusoam]

Interesting note for @Hradec...

What a great research job you are doing!

In true MSer style! (Not afraid to make a phone call, then share the details...)

Have you discovered the initial cause of the corrosion?

Whatever the cause is on one side... did you check the other side for the same issues?

See the comments from Gary above... possible issue caused by old prior work(?)

Wondering if MS has any metallurgists..?  

 

PP thoughts only...

Best regards,

-a-

[Raymond J1]

8 hours ago, Gary0747 said:

I am a Chemical Engineer with an advanced degree  having worked 32 years for Dow Chemical which was  the largest manufacturer of chlorinated organic chemicals in the world.   I know from real life experiences how incompatible these materials are with aluminum. I have to speak up when somebody acting as an expert “corrects”me and makes a bad recommendation that will likely cause harm. Methylene chloride will get into the cracks and joints and over time decompose to minute amounts of hydrochloric acid which will damage the passive protective oxide layer on the aluminum causing exfolation of the aluminum in the crack or joint. Raymond J eluded to caution when talking about cleaning preparation for tank lining above.   I have also felt that some people painting aircraft have not been aware of the hazard here and have used chlorinated solvents to strip the paint aircraft. This type of corrosion will not show up immediately but years later the results can be exfolation damage showing up in the cracks and crevises especially in the overlapping joints and rivets. Yes these solvents are volitale but they can become trapped or mixed with grease, dirt or other materials so don’t just assume the evaporate cleanly like they might in a laboratory beaker.   Do we really understand all the causes of corrosion on our airplanes that we are scraping everyday?  This is just one.  

Gary, you are right, I advocated the use of gel remover and on small areas to precisely avoid migration in the riveting folds, as well as the abundant rinsing with water after stripping. For my part, I dry the washed parts with a vacuum cleaner that can collect water. The goal, as you say, is to prevent the product from seeping into the joints. I cringe when I see paint companies stripping planes with liquid cleaner sprayer.

[Hradec]

A little update.

A guy involved  with the urethane stc reached out to someone that has dealt with servicing these urethane coatings. 

They got back to me and said denatured alcohol or skykleen gel.

I have a sample sitting in a cup of denatured alcohol, no change to urethane.

The other day I had put samples in acetone, paint stripper, and choke and carb cleaner.

The only one that showed any results  is the choke and carb cleaner.

The auto zone cleaner lists the following ingredients 

Acetone,methanol, toluene, msds also lists methyl chloride if it is lot the non chlorinated variety.

It softened the urethane, but as soon as it dries it becomes firm again.

This is not going to work well because of the fumes, flammability, cold hangar, most of sealant I would like to remove is under the top skin.

This thing is kicking my butt. Any ideas are welcome. 

Thanks

Glenn

 

[Gary0747]

Can you remove with a high pressure water blast with out damaging the aluminum?  Pressures up to 10,000 psi have been used to clean equipment in the chemical process industry but the metals are typically harder than aluminum.   There are some serious personal protection issues when using this high of pressure. One guy cut his toes off. 

[Hradec]

1 hour ago, Gary0747 said:

Can you remove with a high pressure water blast with out damaging the aluminum?  Pressures up to 10,000 psi have been used to clean equipment in the chemical process industry but the metals are typically harder than aluminum.   There are some serious personal protection issues when using this high of pressure. One guy cut his toes off. 

It's interesting you mention high pressure water. One guy I spoke to that was involved in removing this urethane system for airforce 707aircraft. He said they are using a 15,000 psi pressure washer and in areas they cant use that the mechanically/manually remove it.

I guess gov contractors have to adhere to strict epa osha's rules so no harsh chemicals. By the way , they are not relocating with urethane after the heavy 707 check, they are going back with regular polysulfied.

The guy I spoke to said to just remove what I need to make my repair, you can feather the new polysulfied sealant over the urethane if you rough it up and clean with acetone. Any opinions on this?

Thanks to all involved in this thread.

Glenn

[EricJ]

5 hours ago, Hradec said:

The guy I spoke to said to just remove what I need to make my repair, you can feather the new polysulfied sealant over the urethane if you rough it up and clean with acetone. Any opinions on this?

That sounds like an excellent plan to me.   Since the urethane seems unlikely to leak and difficult to remove, leaving as much of it in place as possible sounds like a good strategy to me.   If only the affected areas need to be repaired and can be repaired with more common sealant, especially in the areas at the top of the tank where they give less trouble (or at least more manageable trouble), I'd run with that.

 

[Hradec]

Finally got the top skin off.

Used a heat gun, long wooden carpenters shims, small hammer, and a razor knife.

I feel sorry for anyone that has this stc done to their aircraft(polyurethane  coating in the fuel tank).

I am sure it's a bear of a job with the correct sealant.

Now on to replacing the upper spar cap. Aft stub spar is in.

[McMooney]

HMM, maybe co2 blasting ?

[EricJ]

Do you have a plan of how to get that stuff out yet or are you even going to try?

[67 m20F chump]

I wonder if you could find a replacement wing.

[carusoam]

Great pics, Hradec!

What’s next on the list?

Best regards,

-a-

[Hradec]

Didnt look into co2 blasting.

New wing would be buying someone else's troubles. Wings are really long, difficult to transport.

I used a heat gun and wooden shims to get the skin off.

I will attempt gentle heat from the heat gun to get enough cleaned so I can replace spar cap and rivet the skin back on. Was considering not removing all the polyurethane.  I will see how it goes. 

 

[67 m20F chump]

I was able to get a wing from a plane a few numbers newer than mine.  Swapping it is a pain but the quotes I got for spar replacement was more than what I paid for the plane.  
 

I cleaned out the proseal in my tanks but I put in the fuel tank bladder STC so as to never do this job again.  Bladders would get you out of getting off all that old sealant.

[Hradec]

1 hour ago, 67 m20F chump said:

I was able to get a wing from a plane a few numbers newer than mine.  Swapping it is a pain but the quotes I got for spar replacement was more than what I paid for the plane.  
 

I cleaned out the proseal in my tanks but I put in the fuel tank bladder STC so as to never do this job again.  Bladders would get you out of getting off all that old sealant.

I got the plane cheap, and my labor is even cheaper. Just got my pilots license last year. I have my a&p and recently got my ia. Never worked in ga so this is quite the adventure for me. I have met some amazing people along the way. A guy that works at a well reqarded shop as a structure guy has been giving me invaluable tips along  the way.

One day she will fly. I thing 2-3 yrs, still have my day job.

Glenn

[EricJ]

1 hour ago, Hradec said:

I got the plane cheap, and my labor is even cheaper. Just got my pilots license last year. I have my a&p and recently got my ia. Never worked in ga so this is quite the adventure for me. I have met some amazing people along the way. A guy that works at a well reqarded shop as a structure guy has been giving me invaluable tips along  the way.

One day she will fly. I thing 2-3 yrs, still have my day job.

Glenn

The fact that that particular sealant system is known to not leak and you had figured out a way to reseal it definitely makes that an option.   Since you've already had it apart, you can weigh that against the likelihood/difficulty of having to take it apart again and maybe doing something different.    I'm wondering if you can use the traditional sealants on the inspection covers and maybe urethane fill where you put the skin back on, but it's interesting to have multiple potential paths.   The fact that you have time on your side is excellent as well.

Cool project, I'm always interested to see your updates.

[Hradec]

I had to pull the top skin to do the aft stub spar and upper spar cap. 

A big problem with the urethane is I can't buck the rivets with the urethane in place. There is so much of the stuff in places that I think it is reducing fuel capacity.

[Hradec]

The tried and true polysulfied sealants are not perfect but it it can be removed with chemicals that dont harm the aluminum,  easy to repair. Can be scraped off. 

Urethane is great until you need to do some structure work, then you are screwed!