Is this is a static/display model? If it’s going to be a working model, you’ll need to soak the MDF in thinned polyurethane if it’s to stand any chance of being even water-resistant. Do this NOW before you start putting any planking on the model – or reconsider the whole project before you spend any more time on it. Sorry, Michael, but MDF really is the most unsuitable material for working boat-modelling. I restrict its use to stands and transit boxes; even then it’s well varnished.
Dave M