Joanne writes from Bois de Silion in Quebec. She has twice cleaned all the paint off an indoor metal railing, repainted with a primer and a finish coat, and both times, chunks flake off -- sometimes exposing the metal, sometimes exposing the primer. She is not a novice at painting and can't figure it out.
Well Joanne, this is unusual and falls into the category of take-a-guess since you appear to have done everything right. Let's walk through the basic rules for successful metal painting.
Surface prepration: Remove anything that is loose, or strip it down to bare metal. If there is any chance of oils or other contaminates on or in the metal itself, wipe it down throughly with mineral spirits. Make sure there is no dust left on the surface.
The Paint: If you are having problems, make sure you have fresh paint. Some paints can be affected by having frozen at some time in the past, before or after you purchased it. Since the prep work for this is worth more than a can of paint, if you are having problems, buy a new can of paint, maybe even change brands.
All paints have to be completely stirred or you will not get the right mix of chemicals to do the job.
Always use a metal primer. The red primer works better than the white primer, but don't use red under light coloured finish coats. There are some special metal primers which can actually disolve a light rust film, making it part of a well adhered primer to bare metal.
Then use a finish paint formulated for use on metal -- that will be indicated on the can.
Application: Thin coats are always better than thick coats of paint. When you apply a thick coat of metal paint, it can go through all kinds of stresses, even with the metal underneith changing dimentions while it is drying, or after. When you apply several thin coats, it can dry completely and adhear properly. I love the old adage: "Don't 'thin' your paint, just apply 'thin' coats."
Drying: Paint first 'skins', meaning the surface is no longer wet -- but it is not ready for abuse yet. Then paint 'dries', meaning that the solvents have vaporized -- but it is still not to full strength. Then paint 'cures', meaning that the chemical process is finished.
Working Tip: It is best to apply successive coates of non-latex paints either shortly after 'drying' -- usually one or two hours after application -- or after 'curing', but not inbetween the two.
Here's the trick. If you put a second coat on before drying, even if there is a skin, the solvents will soften the skin and you are actually putting on one thick coat rather than two thin coats. If you put on a second coat just after drying, the paint is well attached but not yet cured, meaning that the solvents can bite into the first coat and actually stick better. If you wait more than a couple of hours, you are better off to wait 3 days before applying a second coat for metal paint, and 7 days for plastic paint. You want to wait until the paint is completely cured because inbetween that slightly soft stage and the totally hardened stage, the second coat will not adhere as well. That's funny, but you either apply a second coat right when it says to do so on the can, or you wait three to seven days, but not inbetween.
If the paint still flakes off after all of this, I will have to send this question to our mystery department.
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