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Old 12-05-2018, 10:12 AM
704CH's Avatar
704CH 704CH is offline
Join Date: Apr 2009
Location: Sammamish, WA
Posts: 208
Default Polishing isnít cutting it..

Hey guys, Hope someone who has a expert background in Aluminum Sheet can help me here. I have been attempting to polish with varied success, but have previously polished my wings very well. I live in Seattle and get to polish about every 8 months or so. The wings have developed little pits I guess similar to Pin Pricks that even after I polish (Compound F7) they seem to go away, but shortly as in a few months start to ooze corrosion out of the pit similar to a Petri dish. What are my options?

-Continue polishing harder and deeper to cut through it, then polish more often significantly so like ever 4-6 months? I worry about using F7 and continuing to cut deep.

-Take it to get painted, but then do I still need to do step one above, or will the painting company (Grady?) take care of cutting through the pitted surface and ensure no corrosion shows up under the paint?

Sammamish, WA
RV-7 Standard Build - Tip UP
Flying ! As of 032913
Superior IO-375, WW200RV, Single Lightspeed, SkyView, GTN
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Old 12-05-2018, 01:02 PM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 4,194

I couldn't say whether the pin holes are corrossion or something else like a defect, though my first guess would be corrosion. That said, most that I have seen is surface based and tends to grow across the surface and not deep pitting, like with steel. Others with more experience may add it.

The hole is filling with polishing compound that then over time is reacting to something (likely from the atmosphere) or corrosion below it is causing it to ooze out.

If you have it painted, the corrosion can be stopped chemically, so there should be no need to sand down the pits. Just be sure that your painter takes steps to perform this treatment.

Digging deeper into the surface doesn't help. Typically, once the corrosion starts, the initial layer of corrosion protects against further corrosion. I suspect that the polishing compound left in the hole is helping to trap moisture, which accelerates the corrosion and supports corrosion beyond the initial layer. Guys that leave the blue plastic on their aluminum sees this on the corners or edges where the AL touches the ground in damp environments. It would probably help to clean out the compound out of the pits with mineral spirits after polishing.

RV-6A / IO-320, Flying as of 8/2015
RV-10 in progress

Last edited by lr172 : 12-05-2018 at 01:14 PM.
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