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Old 03-22-2018, 06:54 PM
fcordrey fcordrey is offline
Join Date: Jun 2014
Location: Aulander,NC
Posts: 29
Question Alumiprep33

I have a question reguarding alumiprep 33. If it gets under the vinyl on the skins around rivet holes and dries is the part ruined? Does it weaken the metal?

I have been using it to etch parts before priming. I have not had any problems doing so until now. Last week I prepped my top wing skins and primed them. When I removed the vinyl strips from the rivet lines it was clear that some alumiprep 33 had seeped under the blue vinyl and dried. There were several places where the vinyl was puckered slightly allowing this. Alot actually. It had not done this before on other parts. Maybe I shouldn't do it this way or maybe I did not rinse it good enough this time. I don't know. I will do things differently from now on.

It discolored it some and left a film. Like soap scum on a shower door. I tried wiping it with acetone and mek with no luck. And I rewet it with 33 and rubbed it with a cloth with very little improvement. Then I wet it with 33 and very lightly rubbed it with a white scothbite pad and it did the best but scuffs the shine off like when I prep for primer. The aera I tested showed improvement and I feel that alittle more pressure with the white pads will take off all the residue.


- will alumiprep33 weaken the sheet or will it be ok to continue after letting the etch dry.

- scuff the film off (going to paint anyhow) and continue.

- should I scrap the skins and order new ones and start over.

I've asked Van's and they weren't sure about alumiprep33 long term effects in this case. Suggested I scuff it clean and prime. And carefully watch. I just don't know and would like to know if anyone out there does.

Frank Cordrey #74600
Private SEL/SES
RV-7 Emp Done
Wings 80%
Fuselage started- canoe flipped
Finish kit inventoried
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Old 03-22-2018, 07:26 PM
wirejock's Avatar
wirejock wirejock is offline
Join Date: Oct 2005
Location: Estes Park, CO
Posts: 5,353
Default Residue

I don't think it will weaken the aluminum but I do believe the area is prone to start corrosion so the white residue should be scuffed away with maroon scotchbrite and primer or Alodine.
If you want an environmentally/ human friendly substitute, use Bon Ani cleanser and scotchbrite.
Larry Larson
Estes Park, CO
wirejock at yahoo dot com
Donated 12/01/2021, plus a little extra.
RV-7A #73391, N511RV reserved (3,000+ hours)
Empennage, wings, fuse, finishing kit done. Working FWF
I cannot be, nor will I be, held responsible if you try to do the same things I do and it does not work and/or causes you loss, injury, or even death in the process.
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Old 03-22-2018, 09:35 PM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 7,277

It is an acid. It is made to etch and it did that to your panel. It's intended use is in a time controlled application. Etching can only be stopped by rinsing or the exhaustion of chemicals that create the etching. I don't really understand the desire to keep plastic on the Aluminum sheets. It only introduces risk without upside, with the exception of those planning for polished AL planes. I bought a kit that was 10 years old and the uninstalled panels had corrossion at the edges of the plastic where it was lifting/bubbling. The blue plastic, once it starts to lift at the edges is a perfect solution for trapping moisture and exposing the aluminum to that moisture for very long periods. A parallel is putting a wet bath towel on a piece of steel. WAY worse than pouring a glass of water on it.

The etch must be stopped with a rinse and putting etch on any part or assembly where complete rinsing is not possible or likely is a mistake. This includes assemblies where the etch can pool. The greater the quantity of trapped acid, the greater damage that will be done.

My guess is that it didn't take off more than a thou or so and it's probably fine. You'll have to assess how much material was removed. The crusty stuff is the converted aluminum, like rust is converted steel. You need to get it off to determine how much aluminum is gone. There is no way to convert it back to AL.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 03-22-2018 at 09:45 PM.
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Old 03-23-2018, 01:09 PM
fl-mike's Avatar
fl-mike fl-mike is offline
Join Date: Jul 2005
Posts: 1,733

Get it off now!
It is very important to completely rinse off the phosphoric acid. Leaving it on will lead to corrosion.
Re-treat the area with 33 and scotchbrite pads and rinse well.
As others have said, leaving the plastic on and doing any kind of chemical process is just asking for trouble and makes no sense. Get the plastic off unless you plan to polish in which case you wouldn't be etching anyway.
Mike W
Venice, FL

N164WM RV-6A Slider/O-360/FP Sold 4/2022
N184WM (RV-8) Final stages. Titan IOX-370
N184P reserved (RV-8QB) Empennage
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Old 03-23-2018, 06:28 PM
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jcarne jcarne is offline
Join Date: Jul 2016
Location: Worland, Wyoming
Posts: 2,282

Yep I would rinse it off, re scrub the white areas and rinse again. In the future you may try a different method. For this very reason I don't use alumiprep on any skins. Before I dimple I put a scotchbrite pad on my random orbit sander and go to town. If you put water on it after that you will notice it has the same effect as alumiprep in the sense that it makes it hydrophilic. I have also heard that this is the recommended technique from Akzo but I have not heard that directly. I have had good results with this method (and many others). The only time I use alumiprep is on parts that have tight contours. Hope this helps.
Jereme Carne
RV-7A Flying as of 03/2021
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Old 03-23-2018, 07:18 PM
fcordrey fcordrey is offline
Join Date: Jun 2014
Location: Aulander,NC
Posts: 29

Thanks to those who replied. I don't plan on leaving the vinyl on forever. Just trying to leave most of it in place while I'm still building and handling it. Seems like I can walk by something and scuff/scratch things. This has been a lesson learned and I'll do things differently from here on.

When I first saw there was an issue I stopped and wasn't sure about my next step. Then decided to try and clean it but wasn't very agressive with trying to remove the residue. I contacted Vans and then waited another day or two before posting here. Alot of mistakes and time wasted.

I have serious concerns of the skins being compromised and having increased corrosion problems around alot of rivet holes. Today I decided to order new skins and start over and not worry about the what if....

With that decision made I picked some if the very worst looking places and cleaned them very good. With just my eyes it look ok but with my 20x-40x magnifier there is definite pitting of the sheet where the residue was.

I feel like when dimpled it will crack or crack eventually because of the pitting. So to answer my question it has ruined the skins. And reinforced my decision to scrap the skins.

Live and learn..
Frank Cordrey #74600
Private SEL/SES
RV-7 Emp Done
Wings 80%
Fuselage started- canoe flipped
Finish kit inventoried
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Old 03-23-2018, 08:37 PM
xblueh2o xblueh2o is offline
Join Date: Apr 2011
Location: SF East Bay
Posts: 869

Originally Posted by fcordrey View Post
Thanks to those who replied. I don't plan on leaving the vinyl on forever. Just trying to leave most of it in place while I'm still building and handling it. Seems like I can walk by something and scuff/scratch things.
If your intention is to paint the airplane there is zero need to protect the skins from normal scuffs during the build. Go visit a paint shop. One of the very first steps to painting an airplane is to sand the entire fuselage to scuff it for better adhesion. Paint won't stick to smooth. Pull the vinyl off. It is only there to prevent scratches during the shearing/punching/bending that Van's does.
If your intention is to polish, I would still pull the vinyl off but now you just have to be more careful how you handle the skins.
RV-8 with the Showplanes Fastback conversion
Emp completed except for glass work
Wings completed except for bottom skin and glass work
Fuselage underway
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