Repairing galvanic corrosion holes

Panaman · 10-03-2009, 08:24 PM

I have been researching "aluminum body panel repair" on the internet and stumbled across this site. I am hoping someone can tell me the proper way to repair holes caused by galvanic corrosion (which will be painted). What I have learned so far is:

1. Drill;, rasp; or otherwise (but do not grind) to remove the aluminum which has oxidized to a white powder all around the perimeter until the hole has a 'solid' aluminum edge.

2. Prime the cleaned hole w/ a zinc chromate primer.

From this point forward I have several choices:

3 a. Cover the cleaned and primed hole with 3M aluminum autobody repair tape and spread, sand, and feather a plastic (or aluminum powder based) autobody filler in successive layers; or b. Spread a layer of autobody filler, imbed a piece of aluminum or fiberglass screen and squeege flat. After the filler has cured, rough sand w/ #80 paper to a level surface at the edges. Successive layers of filler can be added and sanded until the entire surface is level and feathered into the surrounding area.

4. Final sand the patch with #320 wet paper to remove any scratches; prime with a sandable automotive primer; lightly dry sand with #320, blow off dust; clean the area with an automotive refinishing degreaser; and tack rag the surface to remove any stray dust particles before applying the finish coat(s).

Any first-hand recommendations as to proceedures, materials, etc. are greatly appreciated.

Tom Nugler · 10-03-2009, 09:20 PM

The first question. What is the application? Is this a structural or cosmetic repair?
This is important because if you’re to fill a perforated piece of sheet metal, especially aluminum, with "bondo" it will fail in a short period of time.
If the patch is big enough to require a backing tape dam and screen reinforcement I would be inclined to replace the panel or MIG weld in an aluminum patch.
If the holes are only about an inch or so, and it’s cosmetic, I might be inclined to try one of the 3M Automix Epoxies. After sanding down to the surface and priming, do a final fill with spot putty.
Find a jobber to help you find the right mix for your application because the 3M catalog is huge. Currently they are changing all the product numbers and deleting obsolete items. Our guy seems to always have to call in for the current number.

Tom

Panaman · 10-04-2009, 11:33 AM

Thanks to Tom for his suggestions above.

I will clarify the issue by saying that the perforations are not structural, only small (+/- 1/2") and cosmetic. The back of the panels in question are not easily accessible which is why I thought using a screen or aluminum tape 'backer' to be advisable so as to give the filler something to 'bite' to over the perforation.

Just an additional note for others: Thoroughly clean all tools which may have been previously used on steel or other metals as using such tools w/o cleaning greatly increases the risk of further cross contamination of the aluminum which is the cause of this type of corrosion in the first place. It only takes a microscopic particle of steel in direct contact with an aluminum panel to start such corrosive action.

Panaman