Tubular structure corrosion in 1994 M20J

[PT20J]

I pulled the interior out for refurbishment and noticed some minor surface corrosion. This surprised me since the later models have the epoxy primer coated tubes and closed cell foam insulation and the windows don't leak and the airplane has always been hangared. The problem is that the primer only works if it is applied to all the bare metal and the factory obviously missed spraying it into the gusseted areas. I cleaned it up and brush applied three coats of epoxy primer. Below are photos and a drawing showing where I found the corrosion. Areas along the tubes outside the gussets that appear stained just have glue residue from the interior installation - no rust.

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tubular structure_20200530_0001.pdf

[MooneyMitch]

Did you use Dremel tool to reach the inner areas of the joints?

[PT20J]

2 hours ago, MooneyMitch said:

Did you use Dremel tool to reach the inner areas of the joints?

I had some .010 stainless steel sheet kicking around and I cut a piece about 3/8" x 4" and glued some 150 grit aluminum oxide sandpaper to it to get into the crevices. I also used a small wire brush. 

[jetdriven]

I have seen a little bit of rust in the gusset in the area right below the baggage door. I think the baggage door seal leaks occasionally, the water finds its way there and it sits in a pool, it  cannot drain. Brush it with some corroseal and replace the baggage door seal.

[Stephen]

Might be a little rust on the gusset, but don't let it fool ya bout what's inside. 

[PT20J]

You get corrosion inside the tubes if water gets in there. This generally happens when there is a window leak that allows water to collect and rust out the PK screws that are screwed into the tubes in a few locations to mount the interior panel attachment brackets. The water gets in the holes and runs downhill and rusts the inside of the bottom tubes where it collects. The most common leak is the pilot storm window. All the PK screws should be sealed.

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[bradp]

Skip what epoxy did you use?

mine is all zinc chromate - so last time it was bunch of PPE in the hazmat can.  

[PT20J]

38 minutes ago, bradp said:

Skip what epoxy did you use?

mine is all zinc chromate - so last time it was bunch of PPE in the hazmat can.  

I don't know. I went to the museum on my field that does a lot of paint work and they mixed me up a cup of their favorite epoxy primer for painting tubes. It's similar to the factory primer in that it has a semi-gloss finish which makes it less porous than most primers. It's probably not the best primer for painting over but good if you are going to leave it exposed.

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[larrynimmo]

This gives me a lot to look for when I redo my interior of my 1981 m20j....everywhere I have removed panels, I have seen light surface corrosion on my tubing...worse than yours, but not terrible...all of my tubing has been coated several times with a zinc chromate paint..I typically clean it up well with sandpaper and scotchbrite, and then I paint it..hopefully I can significantly retard the progress.