Have to fly through rain quite often up here ! Indeed, we had some showers yesterday.
Can't always avoid it around here, but here's the guidelines I use:
- Don't enter the rain unless you can see through to the far side of it.
- Get the windshield heater blower going well before you enter the rain.
- Keep an escape route in sight and be prepared to turn away / back.
- Pick out and keep in sight potential landing spots. (i.e. even more so than usual).
- If conditions deteriorate, act before you lose your horizon ! Turn round or land.
And - as has been said already: CARB HEAT !!
Normally I would not consider going more than five or ten minutes; it is quite tiring - and in a Robbo enough water gets into the cabin that you'd get very soggy !