172 paint and corrosion

[UngaWunga]

So, what's the best way to fix this and prevent it from happening again the future? Plane has been sitting outside for a while. Cessna 172.

Strip the paint chemically, lightly sand it, prime it with something, and spray bomb paint it? Suggestions on chemical strippers, primers, and where to get color matched spraypaint?

 

[Dan Thomas]

The paint is blistered because the metal underneath it is corroding. The paint needs to be stripped and all the corrosion byproducts removed. If the damage to the aluminum isn't too serious, it gets an acid etch and then alodining to arrest further corrosion, then a zinc chromate prime coat, and then the topcoat.

Using a spray bomb will do about as much good as a spray bomb on a rusty car. A waste of money and time.

There might be some serious corrosion inside that structure too, and the whole airplane needs a close examination.

Dan

 

[rbridges]

agree with Dan. It's not the corrosion that you see, it's the corrosion you don't see that worries me.

 

[James331]

Lazy slob

Looks like some old hangar rash that someone did a crap job of reprinting.

I wish people would ether do the job right, which doesn't take much more effort, or just leave the damage be for someone who will do the job right.

 

[UngaWunga]

There's no a lot of it, but enough to have me concerned. Its on the trailing edges of the flaps, ailerons, and stabilizer. There's also a bit on the wing fences (stol kit).

acid etch
alodine
zinc chromate prime

suggestions on where to get this stuff? I've heard of acid etching primers, but not the other stuff

 

[James331]

Aircraft spruce or your local semi high end paint shop

 

[TUPilot]

agree with Dan. It's not the corrosion that you see, it's the corrosion you don't see that worries me.

+1. I'd really want to see the inside of that structure before spending a lot of time repainting it!

 

[Tom-D]

Lazy slob

Looks like some old hangar rash that someone did a crap job of reprinting.

What makes you think that ?

This is pretty typical of the old Cessnas, trailing edges are where the last drop of rain evaporites leaving all the contaminates of acid rain.

 

[Tom-D]

+1. I'd really want to see the inside of that structure before spending a lot of time repainting it!

You can do that,, its just a matter of pulling about 200 rivets, rolling the skin back. Then re-riveting re-painting and getting the repair approved on a 337 and signed off by a A&P-IA.

 

[TUPilot]

You can do that,, its just a matter of pulling about 200 rivets, rolling the skin back. Then re-riveting re-painting and getting the repair approved on a 337 and signed off by a A&P-IA.

Or you can get a decent borescope and look in through one of the openings! Might not be as accurate (nor as lucrative for the A&P) but tends to give you enough of an idea of whats going on to decide if more work is needed.

 

[UngaWunga]

Time to schedule a pre-purchase inspection. Thanks.

 

[Dan Thomas]

You can do that,, its just a matter of pulling about 200 rivets, rolling the skin back. Then re-riveting re-painting and getting the repair approved on a 337 and signed off by a A&P-IA.

On many 172s you can push the rudder to one side and get your eyeball looking right outboard into the elevator. A few had a small root rib to close off that opening. Made it harder to see the bird nests that are often found there. Another bad spot for nests is in the left side of stab, at the elevator leading edge where there's a gap in the LE like the gap on the RH LE for the trim rod. Birds can get in there. And birds make serious corrosion.

Dan

 

[Tom-D]

Or you can get a decent borescope and look in through one of the openings! Might not be as accurate (nor as lucrative for the A&P) but tends to give you enough of an idea of whats going on to decide if more work is needed.

The most "Decent" bore scope has a 6MM camera on the probe, that will not fit the diamond opening in the trailing edge of the cessna wrinkle skins, and the front is closed by the spar.

 

[James331]

What makes you think that ?

This is pretty typical of the old Cessnas, trailing edges are where the last drop of rain evaporites leaving all the contaminates of acid rain.

Maybe it's my screen but it looks like a repaint on the affected area.

 

[TUPilot]

The most "Decent" bore scope has a 6MM camera on the probe, that will not fit the diamond opening in the trailing edge of the cessna wrinkle skins, and the front is closed by the spar.

Looks like an elevator to me... there are a few openings on there that work, obviously the diamonds will not. It might be necessary to remove the elevator to gain access.

There are probably variations by model years, but I know it can be done on 172Ps.

 

[UngaWunga]

Maybe it's my screen but it looks like a repaint on the affected area.

Its not your screen... owner tried to repair/repaint the area. Didn't hold.

There's a few other things about this plane that are just screaming at me to pass it by. There's a section of the flap where two cracks have been stop drilled. Crack hasn't gone past the holes, but the top to the flap has a small section of aluminum that has been riveted on, over the original skin, at the trailing edge of the flap.

And the plane isn't cheap enough for me to consider repairs to all this and repainting, with the engine and avionics in the state they're in.

So I'm broadening my search. Thanks all.

 

[Dan Thomas]

Its not your screen... owner tried to repair/repaint the area. Didn't hold.

There's a few other things about this plane that are just screaming at me to pass it by. There's a section of the flap where two cracks have been stop drilled. Crack hasn't gone past the holes, but the top to the flap has a small section of aluminum that has been riveted on, over the original skin, at the trailing edge of the flap.

And the plane isn't cheap enough for me to consider repairs to all this and repainting, with the engine and avionics in the state they're in.

So I'm broadening my search. Thanks all.

Cessna permits as many as six stopdrilled cracks in a flap skin, as long as they started at trailing edge rivets. The small patch on a flap isn't a big deal either, but on any other flight control there are some strict guidelines in the SRM.

I would expect a cracked forward stab spar, right in the middle at the lightening hole, and I would also expect cracked aft doorposts, right where they meet the gear box. Those are weak areas.

Dan

 

[Stewartb]

Exterior surface corrosion is related to inadequate prep and finish on the exterior surfaces. I hope your mechanic is inspecting the inner wing and other critical areas for corrosion since your plane is exhibiting visible corrosion. You can treat the airframe with Corrosion X or a similar treatment to slow concealed corrosion but strip and paint is the only sure way to stop exterior corrosion. And don't blame it on parking outside. My 180 has parked outside 24/7/365 for 40 years in Alaska and it doesn't have a speck of corrosion on it. It does have a good paint job, though. Paint jobs are not a place to economize.

 

[bluerooster]

#1 get a copy of AC 43.13. and follow the directions.

Pretty much abraid the surface to remove the corosion.

 

[Tom-D]

#1 get a copy of AC 43.13. and follow the directions.

Pretty much abraid the surface to remove the corosion.

Be careful what you use to do that..

 

[Dan Thomas]

Be careful what you use to do that..

Yup. Sandblasting can wreck an airplane real quick. It peens the metal and stretches it.

Dan

 

[Tom-D]

Yup. Sandblasting can wreck an airplane real quick. It peens the metal and stretches it.

Dan

Yep, and stripping it, the EPA hates ya

 

[Henning]

Yep, and stripping it, the EPA hates ya

Have you tried UHP water jet? I'm not sure if it's too strong for light airplanes, but it works great. Shipyards have gone to it because they collect all the strippings in a sump, and then only pay the hazmat disposal fees on x pounds of stuff instead of x pounds of stuff and 5 tons of sand.

 

[Tom-D]

Have you tried UHP water jet?

No. simply because they are mega bucks machines based upon the water jet cutting technology. The Cessna .016" aluminum skin will take about, .00001 seconds to cut thru.

 

[Henning]

No. simply because they are mega bucks machines based upon the water jet cutting technology. The Cessna .016" aluminum skin will take about, .00001 seconds to cut thru.

The only technology they have in similar is the pump, they use a fine spray head though rather than a cutter. They'll not cut through the metal, but they may well do what the sand blaster do. I was just wondering if it was tried. I doubt it would take full pressure to blow the paint off of aluminum considering I can often do it at car wash wand pressure.

 

[UngaWunga]

Yep, and stripping it, the EPA hates ya

But why would the EPA hate me makin' a little dancing money on the side? I could use the fat stack of $1s to pay for a paint job...