Engine Enamel Reaction

[Cris B]

Hi Folks

Back in the fall of 2010 I refurbished the aluminum cased gearbox off my front-wheel drive car. It was a mess with 12 years of road dirt, grime and oil on it. I cracked out the POR15 Marine Clean and got to work getting it cleaned up. Then prepped it with Metal ready before painting it up with POR15 Engine Enamel. I was pleased with the result:

My project has taken a lot longer than expected and in that time the gearbox has sat in a sealed cardboard box in the garage. I've now decided to sell this gearbox on having uprated to the motorsports works version. Opening up the box, something strange has happened to the enamel.

What was smooth paint has become bumpy and lumpy. It's as though the alloy is corroding underneath and making the paint bubble up. So my thoughts are this or that the Enamel has reacted with oil residue held in the alloy casing.

Other parts of the box are not affected. It has only affected the top of the gear box where oil and grime would have pooled up. Anyone on here have any similar experiences or thoughts? Would like to get a handle on this before I consider painting up the new box.

 

[MrSnicks]

Aluminium is porous and the oil probably seeped into the pores. When you painted it you didn't get all the grease/oil/dirt/grime out of the aluminum. That is the main reason hospitals don't use aluminum for things like bedpans, the pores of the metal absorb liquids.

I believe I heard somewhere baking an aluminum item before painting helps get the residue out of the metal. Don't quote me on that one.
Patrick

 

[Cris B]

This confirms my thoughts - thanks. I will wait until the new gearbox needs a rebuilt and then get the casing media blasted to clean it up and will not bother with paint.

Hope posting up this issue is useful to others and saves some time and effort.

 

[Big-Foot]

I'm thinking corrosion and moisture underneath.. If you scratched away some of that bubbling, you'll likely find white or black corrosion. I always wash my cast parts with lacquer thinner and warm them up with a heat gun to sweat out any residual moisture just beore painting..

 

[bad_idea]

Castings are notoriously porous, as is aluminum. Sounds like a double wammy.