Go to a GRP works with 2 jars (eg jam jar and a mustard jar) and ask them for a drop of white flowcoat and some activator. Lob the boy a couple of quid for his trouble. (I know that bit will hurt, particularly in the close season)
You won't need much to coat the top of the transom - no point in buying a load of the stuff unless you have a use for it, 'cos it won't last for ever in the pot.
Mask off the area you're coating to give you a nice clean edge where the flowcoat covers the gelcoat.
Wear some disposable gloves and use a cheap brush, which you can throw away after you've finished.