If in fact someone painted something ON TOP OF VC17, your 'blisters' will continue until it 'all' falls off. Virtually 'nothing' can be satisfactorily applied 'on top of' VC17.
That the undercoat is a bright copper color is highly suggestive that it is VC17 (or can also be "Balto-plate", quite similar to VC17).
VC17 is readably soluble in acetone so a 'quick test' would be to soak a rag in acetone, quickly apply the rag to the suspect area and quickly cover the rag with saran-wrap or aluminum foil, etc. to prevent the acetone from 'flashing-off'. If after a few minutes contact of the covered acetone soaked rag onto the 'copper color', when you remove the rag and LOT of the copper somes off with the rag ... pretty much indicates with a quite high probability that its VC17.
Hopefully the 'other' paint is also VC17 but 'high colored' such as blue or black which can 'hide' the copper powder ... so you might want to also test the 'other' color in the same way. But, IF the 'other color' is not VC17, then eventually all the 'other' color PLUS all the copper color will have to be stripped if you want to apply another type of bottom paint.
VC17 is primarily TEFLON + copper powder based ... so nothing is going to adhere to it other than more Teflon/PTFE based material, its that 'slick'.
