magyk,
pine is a difficult wood to get to hold up over the years. It is very soft and very absorbant, making it less stable. It expands and contracts significantly more than other woods.
Things you need to do :
1) First make sure the would is dry. I'm talking about the moisture content of the wood. Originally this wood was kiln dried down to 4-8%, since then it has been absorbing moisture. You need to find a nice dry (low humidity) place to store this for a few days. A couple of low humidity days will take care of this. This is important for a couple of reasons, the most overlooked is that you want your finish to penetrate the wood as far as possible and the drier the wood the more absorbant.
DO NOT PUT IT IN THE SUN. Sunlight will dry out on side of the wood too fast and it will begin or try to cup and crack.
2) Remember, it is important that both sides of the wood be finished in a similar manner (not the same, similar). What I'm getting at here is that over time you do not want one side drying out while the other side isn't, again especially with pine, this will cause issues. You can have different looks inside and out. Remember, even finished wood breathes.
3) Use a finish that penetrates. Pine, more than most woods, will expand and contract with temperature and moisture. The width of the board more than the length (this is why all well built doors use panels). Most surface finishes (like paint) loose their elasticity after a while. This means that after a while the would is expanding and contracting and your finish isn't.
So, with pine I would...
1) Get an good, penetrating, oil based stain like Minwax and let the stain soak into the wood then wipe off the excess.
2) Let it dry in a low humidity environment for a few days.
3) Get a good polyurethane, and apply a thick first coat (you want this soaking in good).
4) Repeat step 2
5) Sand lightly, and apply a normal 2nd coat.
6) Sand lightly, and apply a normal 3rd coat.
Three coats will be enough.
If you believe that the inside must be white, you can by a good white stain or...
Instead of using stain thin a good quality oil based primer (1 to 5) and apply it, let it dry (step 2). If this isn't white enough then apply a thinned single coat of a good oil based exterior. The key here is having the paint penetrate the would as much as possble, you don't want your paint or your polyurethane just sitting on the surface, it needs to penetrate.
If you meant pine plywood (which is much more stable than the wood, the expansion/contraction/cracking issues go away to a large extent) I would feel better about using a good quality primer and paint like Behr. Making sure the wood is dried before applying the primer and the finish. Most paints will break down over time when exposed to salt water, thus requiring some protection. Slick appears to have had good success layering poly over paint, or he wouldn't advise it. My experience with different layered finishes has been sporadic (including poly over paint), with generally good initial results, but not so good long-term.
Hope that helps.
