That appears to be a layer of clearcoat or a shellac sprayed directly onto the frame paint without prep.
The moisture gets under and lifts the coating. 1st photo.
I had a similar problem with my car hood (bonnet) that was repainted. The clear layer didn't adhere to the base paint. After one winter freeze, when melting snow got under and refroze, it cracked and lifted the clear layer off.
If a solvent doesn't work, steam machine may.
That some sort of top coat had been applied without the proper prep had occurred to me as it seems the coat hasn't keyed in with the layer below. Otherwise, it wouldn't be so easy to scrape off when wet. I don't think I know anywhere with a steam machine so it'll have to be solvent or elbow grease.
A big thank you to everyone who has thought about this for me and posted suggestions.
Fred Smedley & unworthy1 - I had to find out what clay bar was. Looks promising.
I'll report back when I've had time to get to work properly on the frame. In the meantime, Thanks everyone!