Rayal: Here is some info about the same (Jarpickle and Varun know about this better than me, they can add some points too)
The Rubberroid is an alternative to the 3M underbody coating that we get in most car showrooms. It is a thick glue like black rubber liquid, to apply this, you need to dilute using petrol.
1. Absorps road noise (noise from tyres etc will be absorped to some extent)
2. Protects the sheet metal from rust etc
3. The coat forms a mild rubber layer below the car and even if the car is not washed for months together the dirt, mud etc wont be able to come in contact with the metal, so no place for rust to begin
4. Also when you drive in rains etc the noise of water splashing inside wheel well is mildly reduced.
5. Much cheaper than paint (PU paint is approx Rs.2000/ltr, this Rubberroid is Rs.180/litre Disadvantages:
1. Takes more time to do it properly:
This layer does not bond with the smooth painted surface if you try to apply a thick layer. Instead first apply a diluted layer, later this diluted layer forms a rough, uneven, sticky surface which can hold the thicker liquid firmly. So the coat will last for a longer time.
2. Adds weight to the underbody of the car: Although it is only 2kgs added, still it does affect your mileage on the long run.
3. This is not required for cars which have Galvanised steel body like (VW cars).
4. A lot of material gets wasted (dripping), if you try to apply using the gun.
5. Not easily available everywhere, I go to Nayandahalli signal on Mysore road (18kms from my home) to get it. Close by shops dont sell it.
Today removed bumper and headlamp, one headlamp screw is troubling so added a few drops of brake oil and waiting for it to finish.