I have built a stone wall with natural stones dug out of the ground. To join the stones I used sand mix mortar. Towards the exposed outside area I left space for finishing like grouting. This pace I would like to fill with very fine mortar, smoother than sand mix. What should I use?

To join the stones I used sand mix mortar

I'm curious what was the sand mix mortar?

Mortar mix. Just on a side note the diff between mortar or cement is the lime ( Mortar)

Mix material: Portland cement, Lime and sand.

Mix a 92 lb bad of Portland to 1/2 bag of lime to 24 shovels of masonry sand. You can brake these ratios down equally. This is a strong mix we use as much as 32 shovels of sand to a bag of Portland to lay brick commercially.

To measure loose sand buy weight to calculate how many shovels you have in a pile. 16 full shovels of sand is about 200/bs

You can tuck point this in or use a grout bag. Let the mortar set up a bit before you tool it off or you will bring up the lime too much and that whiteners the mortar too much,

To get a feel for that do a test joint and let it set up fairly well. When you tool it and it starts to get a very light black hue tooling it then that's burning it. That you don't want. That's the Test limit to tooling mortar.

The best tool for tooling is a slicker or wide butter knife bent

You can also add dye to the mix and customize it.

Don't know how wide the joints will be there but watch for shrink crack after slicking/tooling it. In the summer sun I carry a small pump spray bottle to spray water keep the tooled joints damp. You have to watch too much of that it can also draw the lime out and "Bleach" that area.

If you find the wall is to shady in color then water it down after all is set up for a day or two and that will blanch out the diff in shade hues. Last resort is Acid mixed 3 water 1 acid. Wet wall well spray acid lightly scrub lightly and rinse well.

Go to the masonry supply house to get materials. While there look for a slicker that has no handles on it. It's a bent band steel with a different size width to slick on oppsite ends. To brake it in I use a grinder to soften the edges on the slicker so as not to leave 'tiny Groves" in the slicked joint. Also A good "Horse hair" hand brush will be used after its slicked and sets up for a bit to knock off mortar specks where mortar meets stone.

Any question get back, I'm an old stone real stone mason and not many of us left.