Have you ruled out stripping off the paint and refinishing the wood?

Glidden Porch and Floor paint could possibly do you well if you decide to repaint.

I painted wood floors with them in another house, did well.....I do not believe a clear would be needed with this paint it is very tough when cured, type it in in a search engine, lots of good words about it including some unconventional uses

Keep in mind, proper prep is still the secret to success!

(I am trying to decide for my stairs and upper hallway if I am going to refinish with stain or paint in this house, if I paint this will probably be what I use)

tesmith... what is that "proper prepping" you mention? can you spell it out for me.

We ruled out staining because we thought painting would avoid sanding, but if that's not the case staining may be back in. My concerns with staining are that the planks are surface nailed, so i'm not sure how much sanding we could do. Would we have to sink all the nails in further? Also, with the floor being pine, i'm not sure it would look all that pretty. I dont think of pine as being a typical floor covering. would it have the pretty grain of an oak?

Pine can and will stain well, just another matter of prep I have stained pine floors from new, I have also sanded pine floors bare and reworked/restained and have them come out very well.
Sink screw heads (to avoid tearing up sand paper)
Sand floor (progressive sanding courser to finer papers)
Vac floor, then I clean with rags either wet with denatured alcohol or a mild solvent

Pine I always prestain, I use a mixture of boiled linseed oil and the solvent of your choice (I have used paint thinner but use denatured alcohol most often many say 50/50 mix I usually go about 75% solvent 25% Linseed) Spread this mixture generously over floor, and let dry (visibly dry)

Follow with the stain of your choice and then the clears of your choice.

I feel personally stripping paint to restain would require extra prep as you would want to get rid of all the paint you could from not only the flat tops of the flooring but from the sides between the boards, which is much more work....

Prep for repainting is like that of any other paint job
cleaning well being one of the most important steps, and a light scuffing to improve adhesion....Many are starting to prefere other means But I would still lean to scrubbing with a TSP water mix.....it would seem overkill perhaps but when I am doing a project as this I tend to error to taking an extra step or two but it is to be on the safe side.

If it was me and I was going to repaint

vac floor
sink any bad nail heads slightly
quick TSP washdown
Real quick light sand (not with floor sander just with a hand sander hooked to a shop vac if possible, wear proper dust masketc) More work but if you are concerned with sanding dust, quick rub over with steel wool or even scotch brite would be alternate better than doing nothing. Thing is you are wanting to knock away any gloss/scuff the surface one way or another.
Vac again a shop vac is fine
TSP wash again or clean with light solvents on rags (follow normal rules harsher chemicals, and caution for flammability needs to be observed)
Paint
The Glidden product I suggested I have had great luck with, wide array of colors available, clears are optional
I would think any of the big companies porch and floor paints may work But I researched, and talked to several of the companies and was surprised at the recommendations of this Glidden paint but it has seemed to be a good product the few times I have used it, I will use it again if I do more floors with paint

Either way is a fair bit of work to do it well
I did not go into removing trim or taping trim etc, I would think this would all be common sense

Other people may have their ideas on methods But I have done staining from new, restaining and painting from new as well as repainting and they have all come out decent.

I am just a hobbyist, I have done this for both the satisfaction of doing it myself, but I have also done it as I have not had the dollars to pay to have it done...

If you do it just take your time, do not rush and it can work out fine!

tesmith... thank you so much for the info. I have one question...what is TSP?

Tri Sodium Phosphate
is a white powder chemical available at most any hardware or big box stores to be mixed with water and used for may things

A nonsudsing powdered TSP compound formulated for heavy-duty cleaning. Preferred by painting and cleaning professionals for removing heavy deposits of grime, smoke, soot, and chalked paint. Kills mildew when mixed with bleach (complete directions on package). Use on walls, woodwork, or floors. Cleans and prepares surfaces for repainting. Also recommended for washing away paint remover sludge.

Cleaning

The major use for trisodium phosphate is in cleaning agents. The pH of a 1% solution is 12, and the solution is sufficiently alkaline to saponify grease and oils. In combination with surfactants, TSP is an excellent agent for cleaning everything from laundry to concrete driveways. This versatility, coupled with lack of toxicity and low manufacturing price, made TSP the preferred basis for a plethora of cleaning products sold in the mid-20th century. TSP is still sold, and used, as a cleaning agent, but during the late 1960s in the United States, overuse led to a series of ecological problems.

By the end of the 20th century, many products that formerly contained TSP are now manufactured with TSP Substitutes, which consist mainly of sodium carbonate along with various admixtures of nonionic surfactants and a limited percentage of sodium phosphates.

TSP is commonly used after cleaning with mineral spirits in order to remove hydrocarbon residues.[citation needed] TSP may be used with household chlorine bleach in the same solution without hazardous reactions.[2] This mixture is particularly good for removing mildew.

Although it is still the active ingredient in some toilet bowl cleaning tablets, TSP is generally not good for cleaning bathrooms, because it can corrode metal and can damage grout.[3]
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Use as a Flux

In the United States, trisodium phosphate is an approved flux for use in hard soldering joints in medical grade copper plumbing. The flux is applied as a concentrated water solution and dissolves copper oxides at the temperature used in copper brazing. Residues are fully water soluble and can be rinsed out of plumbing before it is put in service.

TSP is used as an ingredient in fluxes designed to deoxygenate nonferrous metals for casting.

TSP can be used in ceramic production to lower the flow point of glazes.
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Use as a Painting Enhancement

TSP is still in common use for the cleaning, degreasing and deglossing of walls prior to painting. TSP breaks the gloss of oil based paints and opens the pores of latex based paint providing a surface better suited for the adhesion of the subsequent layer of paint.[4]
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Exercise performance enhancement

TSP has gained a following as a nutritional supplement that can improve certain parameters of exercise performance.[5] The basis of this belief is the fact that phosphate is required for the energy-producing Krebs cycle central to aerobic metabolism. However, it is generally considered to be a bad idea to ingest compounds that are sufficiently caustic to dissolve cell membranes. Phosphates are available from a number of other sources that are much milder than TSP. While trisodium phosphate is not toxic per se, it is severely irritating to gastric mucosa unless used as part of a buffered solution.

TSP is an often used standard cleaning/prepping for paint work, it is inexpensive and works well.....follow directions as printed on box
Again available many places here are two examples
http://www.acehardwareoutlet.com/ProductDetails.aspx?SKU=998007334

http://www.sears.com/shc/s/p_10153_12605_SPM161991050P?prdNo=5

You can also purchase in liquid form But I have never tried the liquids, I have always used the powders

The key to the prep is getting clean, you can use many products but you want something that will degrease the surface cleaning it away well and not leave a residue.....hope this is helpful, none of it is rocket science, just mostly common sense. get things clean and dry Then go forward step by step take your time and think out each step, You can be really pleased how it can all work out and you did it yourself!