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Helicopter Roy
08-19-2014, 11:36 AM
Hi Folks;
I am restoring a '56 Cessna 172 and I just can't get the old paint off the wings and struts.
I've tried two different acid based strippers, the last being CPC.
Usually the top coat comes off but those under, especially the primer won't budge.
I tried to "scruff" the paint-didn't work
I cleaned off any residual film barrier with MEK-didn't work.
I tried covering the wet stripper with sheet plastic to hold the solvents-didn't work.

Any suggestions? :)
Roy

Matt Gonitzke
08-19-2014, 03:46 PM
Have you tried Klean Strip KS-3? I have found it to remove just about anything, including really old green chromate primer. Stiff plastic brushes and scrapers are a must for areas that don't bubble and lift well. Some paints just smear around instead of bubbling up.

Helicopter Roy
08-19-2014, 04:33 PM
I'll give it a try. This paint and primer is stuck like bad reputation.
thanks a bunch
Roy

Bill Berson
08-19-2014, 08:03 PM
Most shops use a lightly hand stroked wire brush in the wet stripper, followed with scotchbright to polish any scratches from the wire brush.
I know the wire brush isn't recommended in the books. But it actually makes the paint stick better.

Don't use a wire brush if you intend to go bare polished aluminum.

martymayes
08-20-2014, 05:17 AM
If a wire brush is used, make sure it has SS bristles. Use of Scotch-Brite will also preclude a having a polished finish. Can never get those scratches out.

The least effort paint removal method, especially old paint like that described, will be soda blasting. It's just not practical for a lot of applications.

Mike M
08-20-2014, 09:01 AM
If a wire brush is used, make sure it has SS bristles.

concur strongly with Marty. using a steel brush will embed bits in the aluminum, ensuring future corrosion. ref - navy (now all military) corrosion control manual.

buddyamine swears the best paint stripper he's ever used is DOT 1,2,or 3 brake fluid; takes off anything he's ever spilled it on if it sits overnight, but washes off easily with soap and water. i've never tried to verify. you probably have some in the garage to experiment with, right?

Bill Berson
08-20-2014, 08:25 PM
concur strongly with Marty. using a steel brush will embed bits in the aluminum, ensuring future corrosion. ref - navy (now all military) corrosion control manual.

yeh, I know that's what the books say. But I worked in several shops that used regular wire brushes in the past 40 years. Never had any corrosion. If it were true, the entire airplane would corrode. It just doesn't happen.
Maybe the scrub with scotchbrite removes the metal particles, if any. Then we scrub a second time to etch (phosphoric acid) which also chemically removes grease, oil films. After that a bath of chromic acid (alodine). Do all that and the paint will stick and protect the metal.
I suppose if you are scrubbing hard enough to embed steel than don't do that.

Never use a powered wire wheel.

Bob H
08-21-2014, 08:07 PM
Find stripper with methylene chloride in it. It's used by aircraft companies in-house for paint removal.
Bob H

Matt Gonitzke
08-22-2014, 04:32 AM
Find stripper with methylene chloride in it. It's used by aircraft companies in-house for paint removal.
Bob H

Yup. Klean Strip is one of those...:thumbsup:

Helicopter Roy
08-24-2014, 10:22 PM
I will be experimenting on Monday since I have the day off work. I got a scrub sponge at Home Depot and plan to brush-on the stripper and then scrub the stripper into the areas where the primer refuses to yield.
thanks again
Roy

Bob H
08-25-2014, 03:21 PM
Be careful not to allow stripper to collect in faying surfaces, like sheet overlaps, where it can induce corrosion. Joints must be neutralized or thoroughly rinsed.
Bob H

bdk
08-28-2014, 09:35 AM
Actually some of you have it exactly backwards! You should NOT use stainless steel wire brushes. Use carbon steel on aluminum.

You want to be as close as possible to the other material on the galvanic corrosion table. Stainless steels are farther away from aluminum than carbon steel. You can also see why cadmium plating is used against aluminum.

Stainless steel does nothing to protect aluminum, it protects itself!

http://www.corrosionist.com/galvanic_corrosion_chart.htm
http://www.insertsdirect.com/blog/
(http://www.insertsdirect.com/blog/)

Bill Berson
08-28-2014, 05:00 PM
Yep, I was going to mention that about stainless, but wasn't sure anyone cared about such detail.
Keep in mind the book (AC43.13) is talking about corrosion removal not paint removal. For corrosion removal, it says use a stainless brush followed by 60grit aluminum oxide abrasive paper then 400 aluminum oxide paper.

If embedded metal is really a problem, do you need to sand and clean all the holes that are drilled with carbon steel drill bits?
Do you sand all sheared edges to get rid of embedded steel from the shear? I never heard of this.

But in any case, paint removal doesn't require the same brisk abrasion as corrosion removal. The idea is to just scratch the softened paint just enough for fresh stripper to get under the paint and lift it. And even if there was any metal bits left, it will come off with the scotch bright scrub.