All trains get a "deep clean" every night, the vandalism is very bad and LU has a policy that states any trains that have been vandalised must not go out into service untill they are clean, hence why you see them looking nice and clean.
The LU network gets a overnight deep clean too
I wouldn't say deep clean.
Trains are swept/mopped everynight.
When graffiti is reported, if it is wet, offensive or where acid is used to aid the etching, then it will be sent to depot where a new train will be provided or cancelled. The BTP will be informed and if they are going to attend to look for evidence, then it won't be cleaned.
If it is not serious then the trains will carry on running around.
There is/was a policy that the InfraCo would not offer a train for service if it was graffitied, but they now work on the basis that, a train is better than no train!
Stations aren't deep cleaned each night either. Cleaners work 24/7 on groups of stations and sweeping, litter picking in customer areas and trackside and mopping take place periodically. Other customer areas and out of the way places are done every now and then.
If an area requires cleaning then it should be reported to the line's Fault Report Centre and a contractor will be assinged.