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Epoxy painting on al casting is too complicated and unreliable

We are facing problems in epoxy painting on al castings. We are chromating, applying metal paste, applying putty, baking at 120 deg, apply emery, first coat of og paint and baking at 180 deg, 2nd coat and baking at 180 deg. We are getting lots of air bubbles on finished surface. Please suggest how to avoid air bubbles. Is their any simpler method?

Jayaprakash Khodhanpur Channaveerazppa
engineer - Bangalore, Karnataka, India
2003

You may want to investigate polymer impregnation of the castings. Vacuum impregnation using low-viscosity resins is often used for sealing porosity in compressor housings, etc. Loctite is the leading supplier of these resins, either as elevated temperature curing types that polymerize in hot water like Resinol 90C, or self-curing anaerobic sealants like Resinol RTC.

Toby Padfield
Automotive module supplier - Michigan
2003

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2004

Hello,

Epoxy zinc dust [on eBay or Amazon] would work better in place of Chromating. Which putty do you use? A epoxy paint applied over a synthetic putty will lead to lifting, but blistering seems to be because the putty must not have dried.

Regards,

Nilesh Oturkar
- Mumbai, India

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Dear Sirs,

Your problem appears to be at the initial preparation stage. All aluminium casting have what is called capillaries (did I spell that right?). These are created during manufacture (like tubes within the aluminium casting and contain air). On heating the air tries to escape giving rise to defects in the paint film.

Try this: purchase a product known as etch primer or wash primer in the States. Starting with a cold substrate apply primer according to instructions - allow to air dry (or low bake)- continue as normal.

If this does not work heat another casting to 65-75 °C. Apply primer - on cooling the theory is the primer is pulled into the capillaries - let casting cool and coat as normal.

There are other methods but these can be costly - try this - best of luck.

Terry Hickling
Birmingham, United Kingdom
2004