I bought a 1955 Rambler in Virginia with gross carpet which I've spent the last few days removing along with 4 trillion staples. Underneath is pine floor which I plan to refinish, and which I assume is Southern yellow pine. It appears there's some sort of shellac finish on top, but I have not tested it. It sands off fairly easily using a hand-held belt sander revealing some pretty floors underneath. I have a bunch of questions and would greatly appreciate any tips before I begin the sanding procedure. These are three quarter inch boards that sit directly on the floor joists. There are some fairly large gaps between some boards approaching 3/8 of an inch in only one place, and roughly a 16th of an inch to 1/8 of an inch throughout the remainder of the rooms.

Since there are a million staple holes and a million nail holes and there are large gaps between the boards, would you recommend to trowel fill after the first sanding? I know that since it's winter, come August some of the field will probably mush out when the boards expand, but I don't see it another option because the crawl space is literally directly underneath the floor and I can't leave gaps like this.

Before the previous homeowner abandoned the hardwood and installed carpet, they had refinish the floor at least once and had quarter round down throughout the house, and of course didn't remove it when refinishing. This leaves a roughly 1/8 inch Ridge in the worst spot and roughly a 1/16 inch Ridge through most of the house around the perimeter off of the baseboards roughly 3/4 inch. Is there an easy way to sand this down level with remainder of the floor, because I'm not using quarter round, but instead planned to use a different style base shoe that's thinner.

Any general tips to someone who's never refinished a floor before?

I plan to not stain them, but instead only seal them with Minwax Ultimate Floor finish semi gloss clear.

Thanks in advance.

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It is going to take a lot of work to make the floor look good again. I look upon it as an investment. It's great that you have access to the under side of the floor from the crawl space. The single layer floors that I have worked on all would benefit from blocking between the floor joists so long after you sand the floor and it gets a little thinner with each sanding the top of the groove will not break off creating splinters from normal foot traffic. I use 2 inch wide strips of half inch plywood with a bead of sub-floor adhesive and fasten this up with short sheet rock screws placed between the floor joists. This makes a big difference when you walk on the floor. Of course when the floor joists are more than 16 inches apart it really sturdies the boards and keeps them in alignment when walked on.
A floor edger will take care of the ridge that was under the trim.
The wood patch for the gaps between boards will stay in place a lot better when the boards move in unison with the blocking installed instead of the T&G being the only support.
I recommend top-nailing butt-joints if they are not fastened well with 6 or 8d finish nails before sanding. Fill the carpet tack strip nail holes and top nail holes after you set them with wood patch to match the color of the darkest boards when you wet them with paint thinner. The flooring and varnish that you use will darken over time. I like to stain the floor light brown before finishing, but it takes an expert sanding job to make a stained floor look great.

When you say blocking between the joists do you mean just run 8 ft long 2 inch wide plywood strips parallel with the joists, using the screws to simply hold them up while they glue cures? Or are you talking about blocking between each individual hardwood board perpendicular to the floor joist?

8 foot strips of half inch ply glued and screwed to the bottom of the flooring in between the floor joists. To make sure that the plug is pulled up tight to the flooring you can pre-drill pilot holes a little bigger diameter than the sheetrock screws, but if you use a screw gun with an impact driver you should be good without pre-drilling.