I'm quoting from memory here, but I'm right, so whatever.
Rule 1: If 50% of a vehicle is obscured, the vehicle counts as obscured.
Rule 2: If any part of an infantry model is obscured, the model counts as obscured.
Rule 3: A Thunderfire Canon is a unit of two models, the Canon and the Techmarine.
Rule 4: If 50% of a unit is obscured, all models in the unit get a cover save.
Situations in which the unit gets a cover save:
The Thunderfire Canon is 20% obscured and the Techmarine is 20$ obscured.
The Thunderfire Canon is 50% obscured and the Techmarine is 0% obscured.
The Thunderfire Canon is 0% obscured and the Techmarine is 1% obscured.
Situations in which the unit gets no cover save:
The Thunderfire Canon is 0% obscured and the Techmarine is 0% obscured.
The Thunderfire Canon is 49% obscured and the Techmarine is 0% obscured.
So yes, it is very easy to ensure that the unit has cover. A few things to remember:
1. The Thunderfire Canon is not a barrage weapon. Line of sight is needed, and the barrel isn't exactly very elevated.
2. The Techmarine can bolster ruins, giving the unit a 3+ cover save.
3. The unit still goes down easy to anything with AP1/2 or a high rate of fire. Cover helps, but they are still very very fragile. Remember that any glancing or penetrating hit that gets through cover kills the Canon.