I am at the point of painting all those small, white metal castings that came with my craftsman kits. I have read that immersion in white vinegar is good, but for how long? Is washing with soap and water necessary AFTER the soak? Do I mount them on a board and spray them all at once? Things like this are why I have always worked with wood.[:#]
The purpose of soaking white metal castings in white distilled vinegar is to remove the mold release agent that is still on the metal, and to slightly etch the metal so paint will adhere and not flake off.
I use an old toothbrush to gently scrub the release agent off the castings after soaking in vinegar for only a minute or less, and then thoroughly rinse with water to get all the vinegar off.
Most important of all, wear latex gloves and don’t touch the castings with bare skin. Your skin will leave an oily deposit on the castings that is almost as bad as the release agent you’re trying to scrub off.
After the castings are thorougly air dried, spray them with automotive primer. Without a primer coat, your final paint is not going to adhere as well.
For white metal parts, just wash them with a grease cutting detergent (Dawn) and rinse thoroughly. I mount them on a paint stirring stick with double sided tape (foam tape), prime them with grey auto primer and paint them with acrylic model & craft paints. I have never had any problems with paint adhesion.
Most white metal castings are made with the “lost wax” process (like brass castings) or plaster molds in centrifuges and some are made with special high temp silicon molds. I have never seen mold release agents used with metals as the hot metal would burn it off anyway, but I could be wrong.
The vinegar is simply used to etch metal and is generally used with brass and copper, not white metal, 30-60 minutes is sufficient. Vinegar can react with some alloys used in white metals and create a white crust, so test a piece first if you decide to etch.
Resin casting does involve mold release agents and laquer thinner works well for removing them, always finish with a good wash with Dawn detergent.
I just wash them in dishwashing liquid and water, then air dry.
Next I apply a coat of Modelmaster spray primer, in white or gray, depending upon the final color.
Never had a problem with paint not adhering.
Now we’re at it - if you want really strong adhesion, do you put it in the oven? When? After each paint layer? and for how long, what temperature?
oops double post - sorry