It's hard to say for sure at this distance, but if you can't cut it chemically and it is resistant to abrasion, then it might be some metal artifact of the manufacturing process. Paint/primer isn't that tough.

You don't say what the frame is made from. If it's steel, wash it with wood alcohol to remove all the chemicals that you've used so far. Then, wash it with a metal preparation chemical containing phosphoric acid. You can find such things at Home Depot, etc. then maybe alcohol again or acetone This will give a good surface for painting. Then, because you have an unevenness associated with this thin layer of something, apply a leveling primer. Then either a primer or top coat depending on the paint manufacturer's recommendation. If you have problems with the topcoat smoothness, sand with 400 to 600 wet and dry and re-apply the topcoat.

Make sure to ventilate yourself and protect your skin.