I am using a Delta sander with grits: 60, 120, 180, 240, 320
Two points in relation to this worth noting:
- you're sanding a little too finely here;
- it's worth trying sanding by hand to see if the results are better.
While many people these days do commonly sand more finely it is generally unnecessary to sand beyond approximately 180 grit. And sometimes you can get away with sanding a finished surface to 150, particularly on softwoods1. Most finishing books will cover this, often with examples showing why it's not beneficial to sand beyond a certain grit in most cases.
With rotary sanders (both types) it is generally advised to hand-sand the surface in the direction of the grain with the same grit you stopped at on the sander. And here it might also be the case that you get better results — although possibly only slightly — sanding by hand, since you'll be sanding in mostly straight lines and because the process tends not to generate heat as power sanding does.
I don't think this answer applies to my problem, since I am not getting splinters everywhere due to low quality wood (I think), but I get entire sheets of wood that are splitting. I am using larch wood.
This is exactly the same issue, just with a different species and not to the same degree, that the other Q&A addresses.
While you most definitely can sand softwoods to a smooth finish — this is after all how most wood is smoothed these days! — how well it works is very much dependent on the wood, and as in some other areas sometimes the wood just won't cooperate. This happens all the time in planing, both by hand and by machine, e.g. when tearout occurs; while sanding tends to be more forgiving2 sometimes by itself it's not enough, as here.
What you're experiencing is caused by an underlying structural issue with the wood, and neither hand planing nor scraping3 would be a sure-fire solution. With planing while the surface could be initially near-perfect, the flaking is just waiting to occur again, and can sometimes happen between the time planing is finished and the finish is applied as the surface dries slightly; additionally the actual application of the finish can cause it to recur, especially if it's wiped on. With scraping the scraper's burr can actually be prone to lifting the flakes, so you could quite easily get a worse surface than sanding is producing.
So what is the solution?
As I indicated in my Comment under the previous Question, any finish that dries hard — shellac, varnish or lacquer4 — can help consolidate this sort of thing and toughen up the underlying surface somewhat, allowing the flaking to be effectively taken off by light sanding afterwards. Basically you're sort of glueing the surface wood fibres together!
Commercial "sanding sealers" are sold for essentially the same purpose, and while they may have additives to aid sanding at heart they're just dilute finish (usually shellac or lacquer, because they dry so fast).
Alternatively or in addition, you can consolidate or 'bury' minor surface imperfections under the finish if you're applying a full film finish.
1 And if painting stopping at 120 is often acceptable!
2 Hence its use as the solution to minor tearout in most workshops.
3 Note that many sources state that scraping is not a suitable technique for softwoods. This is not accurate. However, it's not always suitable and this is a case where it would likely not be.
4 Blended finishes such as "Danish oil" or "tung oil finish" can help as well, but they're generally not quite as effective.