Waterproofing a starter motor

Hi guys
Ever since we did Kirton off road at the end of last year every now and then the starter won't engage but normally will, when we replaced the chassis after going to Kirton ( R.T.A. on the way home ) we didn't replace the starter as we only replaced it earlier in the year but with the 90 being stood for a few weeks waiting for the insurance to go through then replacing the chassis over the next couple of months afterwards the starter would only do this every now and then but last weekend and this weekend son took the 90 off roading last week Silverdale and this week Kirton and now it won't start but aswell as being in water at Silverdale and Kirton it has been well jetwashed after both occations and assuming the starter motor is water loged and or the electrics are still wet

I am about to replace the engine and there is a new starter motor going on the new engine but before I fit the new starter is it possible to seal the new starter motor up, has anyone done this to their starter motor's

Is it actually the starter motor or a dodgy earth from off roading?

Try running a jump cable from the block back to the battery -ve to rule that out

Ive had loads of grief with tdi starters and water/mud ingress, they respond well to a strip down and clean out.
In the end I just kept a spare one, sadly defender 200tdi is a pita to change.
The britpart ones arent to well built inside, they do keep on working though.