Hardwood Floors usually do not require a whole lot of maintenance aside from ordinary upkeep such as Dry/Damp Mopping along with some occasional Paste Wax or other Wood Compatible floor product.

Most Hardwood Floors are covered by 2 coats clear finish - Polyurethane - which is an extremely tough product that Lacquer Thinner and many other Paint Strippers can't break through. A Seal Coat is not enough!
We're talking about 2 clear coats AFTER the seal coat has already been applied.

Wood Stains and Finish Products can usually be applied quite easily with either/or a combination of Rags, Sponges, and even Paint Rollers that have VERY THIN Roller Sleeves. SANDING the hardwood floor should be done by a professional contractor in order to avoid irregular and unsightly marks/dents/dimples that can be created with Belt or a Palm held Orbital Sanders. For excessive gaps between the boards, a professional should also be consulted.
Oil based Wood Putty or fast drying, solvent based Wood Dough are generally not products that are meant to be walked upon.

Applying Stain to Hardwood Flooring can be best and most cost efficiently achieved by masking any present floorbase/trim with 2in. masking tape, followed by applying the Stain with damp, but not soggy or wet rags.
Don't worry about potential tape residue! When the floor is finished and dry the residue can be easily removed with an equal mixture of Paint and Lacquer Thinner.
Be sure to keep an eye out for irregular wipe marks or marks that can be caused by the wet ends/corners of the rag that's in your hand.
When such irregularities dry up they're very difficult and tedious to remove.

After 12 hours of drying time a thick seal coat should be applied on top of the stain. This Seal coat should be allowed to dry for a minimum of 18 hours. The applicator tool for the sealer product can be a roller with thin "nap" or roller sleeve, a stain pad, a thin sponge, and also a large, thick, oil brush.
Polyurethane, thinned with apx. 6 - 8 ounces of Lacquer Thinner per gallon will cause the Sealer to dry quicker, without deteriorating the product.
When dry, the Seal Coat has to be sanded thoroughly and a protective mask should be worn while this is being done.
Damp Mop the floor after having it swept. Then make sure that the floor is absolutely dry before applying the next coat of Polyurethane.

GLOSS is normally not preferred for a finishing shine since Hardwood floors are often installed around obvious, bright places such as Windows and/or Entrance Doors. The incoming light can actually result in a distracting glare. Semi-Gloss finishes usually offer a perfect shine that looks fantastic.

The best applicator for a fine, smooth finish is a Lambswool Stain Pad. Rollers should not be used for the final coat since these may leave behind "Stipple" or "Texture" marks that can be faintly visible and even noticeable to the touch after the finish product of Polyurethane has already dried up.

If in doubt - ALWAYS - consult with Professionals. Hardwood is very, very costly but there's usually no charge for professional advice!