Removing Filler

Nitromors will remove it if it's not too thick.

Depending on type and age, yes a blow torch and scraper would help, but watch out for fumes (use with good ventilation) and don't forget that plastic filler is flammable.

I dread to think what nasties the fumes might contain, but I've never had any problems getting filler out using excessive heat and a blunt screwdriver :)

Although most of the filler I've had to remove has fallen out as soon as the surrounding dust, mud or underseal has been cleared away.

If you have a compressor, an air chisel is a very useful tool for this. It's also excellent for removing old underseal. Obviously it's likely to go though rusty metal too, but I work on the principle that anything which is not strong enough to withstand light air-chiselling needs welding anyway.