Which is better to create a vacuum? Air or water?

[robkm]

I have recently used a car dent removal device, which creates a vacuum round the device by sucking the air out however it did not work very effectively. I was wondering if it would be better if the vacuum was placed over the dent, filled with water and air in the vacuum was released through a valve and then use a pump to suck the water out of the vacuum, resulting in the dent being pushed back out.
Any comments or feedback would be most appreciated
Thanks

 

[mgb_phys]

The limit with a vacuum is that you can only apply 1 atmosphere of pressure (15psi) - so you could do the same by putting an extra atmosphere of air/water pressure on the other side.
But the advantage of vacuum is that it holds itself onto the metal - if you used high pressure on the back then you need some way of holding the system against the car.
There is also a problem that getting at the back of the dent is normally harder!

 

[Danger]

You can't beat the good old 'Morgan Knocker'.
You used to have to drill a hole in a panel and attach the slide-hammer with a screw, but the newer approach is to weld a pin to the metal, then grind it off after straightening.