Try some citrus based paint stripper, I used it to strip the PBH build back to bare timber and have also discovered that it takes the powder coat off aluminium windows ( oops )

It's also water soluble so a good rub down with a wet rag afterwards and left in a warm dry place for a few days to dry out, around the house I've painted and stained over the stripped items with no ill effects

The stuff I used came from the green shed