Treating underside of a rotary mower deck to prevent corrosion question

it also necessary to drain the oil ?

All depends on the configuration of the mower. - spark plug facing forward, backward or to one side. Tip the mower with the plug facing skywards so that the oil goes to the back of the crankcase. 

Fuel. Assuming that you’ve just finished mowing and there is only a small amount in the tank, remove the cap and stretch a bit of clingfilm over the filler and replace the cap. That should be good enough to prevent leakage via the cap breather.

Under deck  corrosion . Probably the best prevention is not to put the machine away with a mash of wet grass underneath and leave it to fester in a warm damp shed. The acids in wet grass can eat through even cast alloy decks. A good scrape out after use is probably as good as it gets . However, how many of us do that?  If you really want to go to town on a steel deck you could have it sand blasted and either galvanised or hot zinc sprayed but it would possibly cost more than the mower. In season I’d just scrape it out but after the “last mow” ( and how many times have I heard that”  a really thorough scrape our and treat it with a product such as Dinitrol , Waxoyl, or Ankorwax.

I recommended Waxoyl to a friend on a similar issue, He now uses the product as part of his maintenance and cleaning routine, it comes in an aerosol and applied with the minimum of fuss on clean surfaces, the benefits outweigh the short term application of paint protection. ( Halfords sell it )

Hi - Thanks, I have some Waxoyl so will use that before winter.