Any easy way to remove old set putty please?

[footprints]

Hi, I am refurbishing my cast iron gutters they are in pretty good nick apart from the odd bit of rust and endless coats of paint.
I am removing them, stripping the gutter with a heat gun and making a few small repairs to a couple of external bends.
All going well (so far fingers crossed) apart from the old putty in the joints it is like iron!
I have so far tried, soaking in white spirit, soaking in strong soap powder solution, power wash, heat gun, paint stripper, wire brushing and laborious chipping away with an old wood chisel whilst swearing at it. The wood chisel is the only thing that has any effect, don't think the swearing really helps but it makes me feel better!
Has anyone got a secret solution they will share please?

Just seen some videos that suggest using a steamer so that is next, I will still welcome any other ideas though.

 

[chappers]

as you are doing carefully chip it off

 

[footprints]

Sorted!
Tried strong descaler It sort of worked OK on light deposits (I guess dried putty is a sort of sand/ limestone) fissed and dissolved but no good on heavy or painted areas of putty.
The wallpaper steamer with a concentrated nozzle worked well, hold it on the area for 30 to 60 sec and then pushing a screwdriver in and twisting worked like a dream!