Laying down Sound deadening

When prepping the interior of the vehicle to be covered in sound deadening, such as fatmat etc.
Do you guys strip off all of the factory deadening or can i get away with leaving it on and cleaning it up then layering the fatmat on top of it... its stuck real good all over inside the cabin with no chance of lifting or breaking off in chunks any time soon. its close to 1/8" thick in most areas...

You can leave that factory stuff and layer over it. But make sure you use the detergent FatMat supplies, or a de-greaser detergent like Dawn to clean ALL areas that you will be applying the sound deadner to. Also, the aluminum outter coating can be sharp where you cut it and on the sides, use gloves when working it into the contours that you are covering. I used Fat Mat and am VERY pleased with the product and price.

I stripped all out ,cleaned well, & coated all with Pro -15.Used Fat Mat