RTV is an acronym for Room Temperature Vulcanization
These products are fantastic for replacing gaskets sealing gaskets or replacing o rings and using on metal to metal places where there is no gasket. Basically they seal things and remain flexible.
Vulcanization or vulcanisation is a chemical process for converting natural rubber or related polymers into more durable materials by the addition of sulfur or other equivalent curatives or accelerators.
The RTV we are familiar in using for automotive applications is a one part usually sold in a tube or if you are baller in a pressurized can.
There are many types usually identified and referred to by the color. These have different applications and different heat capacities.
Room-temperature vulcanizing (RTV) silicone is constructed of reactive oil-based polymers combined with strengthening mineral fillers. There are two types of room-temperature vulcanizing silicone:
RTV-1 (One-component systems); hardens due to the action of atmospheric humidity, a catalyst, and acetoxysilane. Acetoxysilane, when exposed to humid conditions, will form acetic acid. The curing process begins on the outer surface and progresses through to its core. The product is packed in airtight cartridges and is either in a fluid or paste form. RTV-1 silicone has good adhesion, elasticity, and durability characteristics. The Shore hardness can be varied between 18 and 60. Elongation at break can range from 150% up to 700%. They have excellent aging resistance due to superior resistance to UV radiation and weathering.
My experience with rtv has been when a seal is needed between two close fitting metal parts. Most recently on a mazda miata 1.8 engine the valve cover and the oil pan. Black RTV is called for and shown below in the red spots there are caps holding the cams in place and RTV is called for to seal between the caps and the cylinder head and fill in caps between the hard rubber valve cover gasket and the caps and cylinder head.
Sometimes repairs are called for when there is no gaskets but a bead of RTV is called for such as in the oil pan. In this case the dropped u shaped parts of the oil pan seal against rubber gaskets installed in in the long block assembly.
and a bead is run around the flat surface of the oil pan that mates to the block and around all the bolt holes.
In closing RTV is awesome at sealing. But it has its limitations as well. Dont be that guy who installs a 1/4" bead around everything. Be sure to read your repair manual and follow its suggestions as well as the manufacturer of your RTV to find the proper type and procedures.