1927 florin wierd toning

[serial]

can anyone tell me what is going on with this coin?

[trout]

Looks like PVC damage to me :ok:

[Robster]

Removed from a biscuit tin yesterday for the first time in at least 40 years ( according to the owner).! Some nice natural toning. :ok:

[stevo1962]

Yeah looks like pvc and it should come off with acetone

[mkonst]

I've been thinking I need to start learning about the different high points on various coins, so I can get a better idea about grading and the first picture to me shows the high points quite well on the 27 Florin (as the PVC stuck to the highest points).
Being new to this and not having done any research yet, am I on the right track with my thinking above?