For the purpose of this Instructable I'm dealing with medium section steel preferably black Iron that has a thickish 1-2mm coating of a plastic that yields slightly to a finger nail as this is probably hot dipped nylon and relatively easy to remove. My starting material is an old steel fish tank stand.
Step 1: Identify
Having cut out the cross bars It occurred to me that there was a good amount of re usable angle iron here about 5m at a rough estimate and at £5/m that is worth saving. The steel work was clearly black iron and the slight yield to a finger nail & rebound, along with the thickness of the coating 1-2mm suggested it to be hot dipped nylon. Electrostatic nylon tends to be a thinner coat and a bit harder and harder to remove.
PPA is a brand named blend of nylon/ polyethylene If you live in the UK the most common place to find it is the grey coating on sign posts also being applied by the electrostatic method it is more suitable than hot dipped nylon for large items(I've coated davits for ships lifeboats with it). The coating on this frame could be white PPA. A thin hard coating that looks like paint but resists paint stripper is probably epoxy/polyester powder coat, tends to be used on kitchen appliances, shelving in supermarkets, computer towers, architectural metal work.. resistant to 200c+ makes a good base coat for other paints in a home setting removable by sanding only so I'd leave it unless welding. the salvage.
Step 2: Removal
With your cutting tool of choice slice a strip of a narrow section,. If this is easy the rest of the job will be to if it comes off in short sections the rest of the job is going to be a lot more blade work, if it comes off in small brittle chips it is probably powder coat don't bother trying to remove it.
Step 3: Peel
Having cleaned the whole edge with the blade it should be possible to peel the coating off the wider sections, whilst I was able to lift the first section with a finger nail it may be necessary to use the blade to raise a tab large enough to grip and pull on.
Step 4: Tough Bits &why
The oxide layer of the black iron is what makes it easy to remove the coating. Where this has been removed to facilitate welding the oxide layer has only been formed by the heating prior to coating and the coating adheres to the metal better, and it grips mechanically in the grinder marks. It is still reasonably easy to remove with a blade. Similarly electrostatically applied nylon or PPA on bright steel is going to require a lot of blade work especially if an etch primmer has been used, as the metal is not heated until after the application and the coating starts to fuse before oxidisation can take place. Safety note in several photos I appear to be cutting towards my leg with a sharp blade! These are staged, shot for illustrative use the blade isn't actually being pushed. When cutting plastic off metal rest the metal on a wooden block and cut towards that.