The recipe for stain:
- a lot of thinner
- a little bit of varnish (binder)
- some colorant -- dye dissolved in the thinner and/or
pigment(powder) suspended in the liquid.

The dye soaks into the wood fibers and colors it. The pigment lodges in the grain structure. The varnish is there to hold the powdered pigment in place until it has a finish coat applied.

Now here's the problems:
- To get darker with the same color dye, you need a higher concentration, not repeated applications. Since you are not mixing your own dye from scratch, you cannot control this.

- In order for the pigment to color, it has to have somewhere to "lodge." With every application of stain, you are adding a bit more varnish and sealing the surface more and more, so there is less for it to lodge in.

- The pre-conditioner is essentially the varnish (or drying oil) and thinner parts of the stain without the color. Its purpose is to seal some of the pores so that the adsorption of the pigment is more uniform.

- Too many applications of stain can affect the adhesion of the top coat, and in some cases the clarity of it. It's time to stop staining now.

One solution might be to start with your first coat of finish. Let it dry. Sand, then apply a gel stain as a glaze. This will provide some overall darkening. Let it dry, then apply more coats of finish to seal it in. While you can't go from 3 to 8 (on a scale of 10 darkness), you can get more color in there this way. You can repeat this a couple of times, as long as you don't start getting opaque.

Another common professional solution is to use toners. But unless you have the ability to spray, this is not a practical solution for you.

Another problem down the road is that clear finishes don't work all that well in exposure situations (which I am presuming a storm door is an exterior door). An alternative to varnish is to use a deep base oil trim enamel. Deep base means it has no colorant added, just paint. It has the properties you want in an exterior finish. Don't let the apron wearing guy tell you he has to add pigment to it. Just use it straight and it will dry fairly clear.

Two options I recommend against:
- The all-in-one finishes such as Polyshades. This is a terrible product. I just had a phone call from someone that was in a similar situation and tried this. It turned out so bad, she had to strip and start over.

- Products called "Spar Urethane" These are not very resistant to UV and will likely fail within a year given a moderate sun exposure.

see the link if you want to try this.

First rule of finishing : practice on scrap.

Here is a link that might be useful: http://www.popularwoodworking.com/features/finish6.html

if not spar urethane, then what? Thats exactly what the guy who made the door told me to ge...

You have two good choices in exterior varnish.

A good marine varnish. Epifanes, Pettit, Interlux and a couple of others whose name escapes me. These are quite expensive, $40-50 a quart and available only at shops that cater to boat owners & builders. These will need regular maintenance.

An oil-based exterior trim paint, deep base, without any added colorants. You can buy these at the box stores and hardware stores, if you ignore the guy wearing the bright colored apron or vest.

Here is a link that might be useful: using deep base as a clear finish