underseal/painting

As part of my (ever increasing) restoration project I have finished most of the welding work and am moving on to the 'orrible task of tidying up underneath the vehicle ready to paint and underseal. Have made a start on all exposed metal areas in the cab with hammerite smooth.
Suggestions on the easiest way to remove the original bitumen underseal please!!
Have messed around with various methods from using a blow torch and scraping, using solvents, but the most successfull so far has been rotating brush + drill approach. Thing is I've spent well over 10hrs aleady doing this resulting in getting covered from head to toe in black ****e(even in my ears and eyes!) and still have a long way to go.
I have bought a big can of hammerite underseal to cover the chassis and under the floor, according to the can you need to remove all loose rust, paint and any grease before applying. If I concentrate on brushing down all corroded/rusty/loose/damaged areas would the new coat adhere to areas of original undercoat or do I need to completely strip the whole underside down to metal? I hope not!!
Finally, are there any areas I need to avoid in order to keep the mot man happy? I know that underseal near brake components would not be a good idea but was thinking of painting brake lines and calipers with hammerite

I took mine to a locla place that waxoils trucks and busses and paid them to do the very dirty work of getting rid of most of it. Only cost me something like £40-£45. Worth every penny IMHO.