I had that problem here with my first needle scaller, it lasted like 5 hrs.
But I bought it at Princess auto here in the Toronto area and there return policy is great.
They say you can bring back any tool anytime for any reason, so I did. The second one is a much better construction then the first.
Also I would only use it to take of the hard black under coating. I added some rubber foam to the handle and body to absorb the shock to my arms, it can cause nerve damage if you do it for a prolonged time repeatedly.
If I could have gotten it media blasted I certainly would have.

when I remove the undercoating Do I have to spray the bottom with anyhting so it doesnt rust?

how much does all this undercoating weigh?

i had done this back on my duster last summer. not a fun job. i actually haven't worked on the car since because it really tested my convictions to complete the car as well as opened up a whole new can of worms.

after removing all the undercoat, i am now going to have to paint the whole underside of the car. while i have to do that, i decided to paint the engine bay as well since i have the car all apart, why not i figured.

i used the following methods:
- torch and scraper
- chemical peel (some old rust bust from the 60's) and scraper
- younger brother + the above methods

i would recommend using a plastic scraper if possible, as this would eliminate any unintentional gouging of the metal. if you use a metal scraper, file down the corners first.

I used a torch the outer curved portion of a linolium n floor cutting tool for the inner fender wells. Worked well on the compound curves instead of the squared putty knife.

We use dry ice blasting to remove undercoating. It can be done without removing the undercarriage parts since dry ice sublimes completely.
I have even removed all the paint from my 1958 Corvette without hurting the original Gel Coat layer.

not one of my pictures but it shows nicley what can be done with dry ice blasting (Alfa Romeo).

Links for the US: http://www.blastcleaningdirectory.com/Countries/USA.htm

How it works:
Cleaning with dry ice! This new development is quickly expanding around the world. One system uses small rice size pellets of dry ice shooting them out of a jet nozzle with compressed air. It works somewhat like sandblasting or high-pressure water or steam blasting, with superior results. The frigid temperature of the dry ice -109.3°F or -78.5°C "blasting" against the material to be removed, causes it to shrink and loose adhesion from its sub surface. Additionally when some of of dry ice penetrates through the material to be removed, it comes in contact with the underlying surface. The warmer sub surface causes the dry ice to convert back into carbon dioxide gas. The gas has 800 times greater volume and expands behind the material speeding up its removal. Paint, oil, grease, asphalt, tar, decals, soot, dirt, ink, resins, and adhesives are some of the materials removed by this procedure. Only the removed material must be disposed of, as the dry ice sublimes into the atmosphere.

Replaces Sandblasting:
This method is superior to sandblasting because the dry ice is soft enough not to pit or damage the underlying surface. The dry ice sublimes quickly into the air and only the removed material must be cleaned up. Dry Ice blasting eliminates equipment damage in two ways. First, dry ice does not erode or wear away the targeted surface as traditional grit media and even wire brushes do. This means that surface integrity and critical tolerances are preserved and equipment will not have to be replaced due to surface erosion common with sand, glass beads, and other abrasive media. Second, with traditional cleaning methods, equipment is often damaged (bumped, dropped, etc.) while in transit to or from the dedicated cleaning area. Instead most equipment and machinery can be cleaned while in place.

wait till the global warming people get a load of this.....

We are using dry ice to blast clean our sanding belts. It really is pretty cool. As stated, rice size pellets, nozzles mounted on a track right in the machine. It vaporizes when it hits so there is no media mess. Not sure what it would do with rust.

My car had sat from 1974 til now.
I worked under my car- on axle stands- for 21 months.
I used torch and scraper,wire brush on my drill to clean up,and hammer and an old beat up flat head screwdriver to chip away the brittle stuff..
I found a lot of surface rust under my undercoating so I had to remove it..I cleaned up it to bare metal,a coat of expoxy primer and then painted. Some areas were pitted already and I treated them with phosphouric acid and/or a rust convertor.
Not perfect,but I know it will never see snow or salt and will last a long time.

2006:

2008: