I'm not singing its praises as a fixing method, but three screws from the outside also works. You need to drill into the rainboard at an angle to the surface which is a little tricky to start ( use a pilot drill ), then fill the holes over with filler when the screws are in. It works, but is visible if you are using a clear varnish. Remember to seal the join of the rainboard to the door to stop water sitting in the join ( frame sealant ). Also remember to treat your softwood with anti-rot before you paint it.
Andy.