I’m looking for a bit of advice, I have a wooden white exterior back door. I am looking to eventually paint this with weathersheild Exterior High Gloss Conker. Firstly to sand the wooden door down, would 120 grit sandpaper be ok to use? Secondly, what undercoat would you recommend using on this prior to the high gloss? Apologies if these questions are simple, it’s just I’ve never had to do this myself before? any help would be appreciated. Thanks.