Lacquer -Stik is what gunsmiths and military armorers use to highlight serial numbers and sight markings.
Once dry, Lacquer-Stik is extremely durable and more or less permanent.
To apply, as above, use something to degrease the lettering and the area around it. Lacquer thinner, paint thinner, or alcohol and a solvent proof toothbrush works well.
Use a knife to scrap the end of the lacquer-Stik to expose soft, fresh paint and rub it into the lettering, making sure not to leave voids under the paint in the lettering, and making sure the lettering if filled completely.
Allow to dry a few minutes then remove the excess. To do that, use small dabs of lacquer thinner on paper towel to wipe off the excess.
Change the paper towel often to prevent smearing paint back on the surface.
Be careful not to use too much thinner and be careful not to wipe the paint out of the lettering.
As above, Q-tips and thinner work well to get the "ghosting" of paint off near the lettering.
Allow the paint to dry a couple of hours for best results.
On steel you then need to apply a light layer of lubricant on the area to prevent rusting.
On aluminum you may need to also use some lube to restore the look of the aluminum to match the rest of the rifle.
Once the lacquer-Stik fully dries and hardens it'll last forever unless you accidently wipe or brush it out with a solvent while cleaning the rifle.
If you want to remove it, lacquer thinner and a toothbrush will do it fast.