Do NOT use WD-40 on the inside of your engine! After Run will absorb the water in the engine, not just displace it.
Pretty sad that your fuel has water in it to start with
I have used WD40 for many of the 20 years of using nitro motors and still can't fault it, some motors have been stored for up to 12 months after use and still as fresh as the day I packed them away.
The main reason I flush my engines with WD40 is to wash out any remains of Nitro (very corrosive) and methanol(very hydroscopic).
Has any one tried air tool oil, will not absorb moisture and gum up as mentioned earlier and will look after rubber seals etc