With corks and foams I use a mix of different acrylics and latex. I prime with Kilz at least two coats.
Plastics are different because there is normally a reason why the paint has come off. To prep a plastic for re-paint I do the following:
1. sand down the decoy to remove any loose paint
2. "Flash Over" the entire decoy. Done with a small propane torch run over the decoy. Not enough to melt the decoy but it will burn off any remaining mold release which is normally why the original paint wont hold.
3. wipe down the decoy with an alcohol based cleaner and let dry
4. prime/Paint decoy with a paint like Krylon fusion.
Have you noticed that common sense is not as common as it used to be???