Hi Spud,
Lovely looking model, well done.
In answer to your query, it is a good idea to give the whole hull a few coats of the varnish with the first two or so thinned by up to 50% with its recommended thinners (Should find details details on the tin). These first coats will stiffen the microscopic hairy fibres of the wood and allow them to be sanded down to a very smooth finish. Doing this also ensures that the active part of the varnish is carried below the surface of the wood itself making the surface stronger and with deeper protection. Careful sanding with fine grade glasspapers or wet and dry should make it feel like a Talcum coated babies bottom! Wipe of after each sanding with a tacky rag or a slightly damp cloth and if it is not a water based varnish allow it to dry thoroughly before the next coat.
Yacht varnish can be a bit thick and treacly and a bit of thinners (about 20%) in subsequent coats will help it go on thinly.
Remember that if you want a deep mahogany finish on the upper pats of the hull then this should be stained before you apply any varnish. Make sure there is absolutely no glue on the outside of the hull or it will stop the varnish penetrating and give you a paler patch.
Some of the Acrylic based varnishes are very good now, water soluable when wet and waterproof when dry. They are a milky colour when you start, that goes away as it dries. Acrylic Floor varnishes are very hard wearing and can be cheaper in the long run. Exterior grades of varnish include UV protection as well. (I only use Acrylic varnishes on my models now, and apply them with a foam pad type brush)
To get a really nice finish (glass-like) you will need to apply anything up to 10 coats of varnish with a fine rub down using something like 400 grit wet and dry paper, used wet. Your cream lower hull will cover the varnish where you need it but whatever you do, do not attempt to use a cellulose product as this will damage your lovely finish. A final thin coat of varnish over the cream and the clear will seal the join and any coloured trim you apply,,,,,It should look stupendous.....Good luck.
Ron.
PS This weather, bring it indoors to dry, its now getting too cold and your varnish or paint will not dry glossy.