I am trying to re-stain a door here. I sanded and applied pre-conditioner. Ofcourse as you can see from the first picture, I had to remove the original stain in some areas. These areas had marks that I needed gone from the wood. When I apply my first coat of stain though, and then wipe off the excess, I notice that the parts of the door were the original stain was left on, does not seem to absorb any of the new stain. What can I do to change this behavior?
In general, finish coats will not absorb any stain. The finish seals the pores of the wood preventing anything from penetrating.
True stain can only be put on wood that does not have a sealer on it. Where you sanded, you removed the sealing finish coat. That let the stain penetrate and darken the wood more than the surrounding areas.
The only real fix now is to sand off the finish from all of the visible surfaces, pre-condition, stain and refinish. You may have to do a little blending or feathering on the edges of the areas that have already been stained.
The only alternative is to use a poly-stain. This is really just a heavily tinted surface coat rather than a penetrating stain. You would need to make the color much darker to cover the variations you now have. Probably not the best idea.
I agree with James. Use paint stripper, and give it a thorough rinsing. The can of stripper will have the instructions on rinsing. After that, then have at it with the sanding. The reason being, the finish will sand differently than the bare wood and the bare wood will lose, dishing it out slightly before the finish is removed. The stripper will allow the sanding to occur over the whole surface evenly. When reapplying stain, if there is any stain left on the surface, the stain will appear blotchy. A second coat of stain will even it out. Your doors are pine, really do not need a pre-conditioner IMO, use a stain with a light pigment base. The original stain looks like it was a light base. You new stain looks like a dark base. That's ok too if you want the doors to be darker.
The pre-conditioner, when applied, if all the old (darker) stain is not gone, it will allow the new stain to take differently (blotchy) in those areas.
Re: the doors solid wood or thin hollow core doors? Weight when unhung and picked up will tell. Also deflection when pressing pretty hard in the center section will reveal a hollow core door if still hung on it's hinges. If you want to strip a wooden door and it is solid wood, sanding through the stain and paint is the way to go as you can start with 80G abrasives and really move fast with stripping. When done if paint or stain is still stuck in the dings and depressions, spot paint remover is a good idea and then fill with wood dough filler and sand off high spots. For really heavy paint and stain stripping, a gel paint remover can speed things up. Older paint check for lead with test strips FIRST!
If your door is hollow core, then no paint stripper allowed and be careful sanding as these doors may have really thin pine wood skins that you can burn through into some kind of foil or plastic MDF laminate. Paint strippers will soften the adhesives used to hold the door skins together and the gel strippers are notorius for softening industrial adhesives as they penetrate through wood and polymers so quickly. Best bet is to use a solid stain or paint for 100% coverage and paint all the interior doors the same color stain as interior hollowcore doors are practically disposable.