Rog Smith:
You can put on another coat(s) of Penophin, or you can opt for a marine (also called "Spar") varnish. Spar varnishes were originally intended for use on the wooden masts of ships, but they're pretty well used on all the non-painted wood you find on boats because of the continuous exposure to the Sun and the weather.
You should also be aware that oil based paints are going the way of the dinosaur, and you may not be able to buy oil based coatings in the near future. In that case, I expect you could simply give your door a light sanding and apply a water based marine varnish.
You don't need to know the rest...
I expect that's an exterior stain on your door, and as such it will have UV blockers in it. UV blockers in wood stains typically consist of yellow, red and brown iron oxides which are transparent to visible light, but opaque to UV light, and that's how they protect the underlying wood from the Sun.
Penofin stands for PENetrating Oil FINish, and all drying oils (like Linseed oil, Tung oil, Poppyseed oil, Walnut oil, etc.) have molecules small enough to penetrate into the wood some distance. However, the oil doesn't need to penetrate into wood to dry to a solid film. So, if you're simply wanting to increase the UV protection on your door, I see no good reason why you couldn't simply apply more coats of the same Penofin exterior stain.
Interior wood stain is nothing more than dyes dissolved in mineral spirits or alcohol. When you put the stain on the wood, the mineral spirits or alcohol is absorbed into the wood cells, and the dye goes along for the ride. The mineral spirits or alcohol then evaporates from the wood, leaving the dye behind inside the wood cells and wood cell walls. In this way you can darken the stain on wood by allowing time for the mineral spirits or alcohol to evaporate between coats of stain, so that with each application of stain you get more dye into the wood.
The primary difference between an interior wood stain and an exterior wood stain is that exterior wood stains will have some "binder resin" added to them. This is the stuff that forms the transparent film on the surface of the wood. So, if PENetratingOioFINish is true to it's name, the binder added to your exterior stain is either linseed oil, Tung oil, or maybe an alkyd resin. The purpose of adding that binder resin is to encase the UV blockers and mildewcides which are added to exterior wood stains. So, when you apply an exterior wood stain to a piece of wood, the mineral spirits or alcohol is absorbed into the wood (and since the dye is dissolved in that mineral spirits or alcohol, it gets absorbed too. The binder resin, in this case a drying oil (or maybe an alkyd resin) will get partially absorbed, but most of it will stay on the surface of the wood as a wet oily film. As that oily film reacts with oxygen in the air, it solidifies, and the UV blockers and mildewcides in the stain get encapsulated in that solid film very much like the raisins in raisin bread.
If you put another coat of Penofin on, all that's gonna happen is that the mineral spirits or alcohol is going to evaporate into the air, and the dye in that second coat is going to end up encapsulated in the binder film just like the UV blockers and the mildewcides. The dye in that film won't do any good nor will it do any harm. But, the important thing will be that now you have two layers of iron oxide particles that are opaque to UV light, thereby doubling the UV protection on the wood.
Alternatively, you could just go with an oil based or water based marine varnish.
Hope this helps.
Post again if you want to know the process by which drying oils like linseed or Tung oil react with oxygen to form a solid film.
