Never got used to it myself. Know the technique, couldn't master it. Should be a normal thing to find given even my Mom has one... Go to a real paint store perhaps?
Me, I like really crappy paint brushes and even brooms at times. This or an extra soft touch with dry brushing wood grain.
Never also got good at wet blending my wood grain though if you want a really good and lively one, wet blending is probably the way to go. This given a balance between clear
gloss and semi-gloss layers in doing standard graining and the real mixing and blending that goes on with a wet grain...
Have fun with the paint technique. There is also the graining combs and other similar ways to add a grain but I understand the technique if useful for doing the swirls of grain with the asked for tool.
Question than becomes does your grain become swirls or is it for the most part parallel with some perhaps movement in it's grain lines?
Old fir lumber for the most part wouldn't most likely have such cut lines, it would be parallel in this cut being taken from a section of the lumber that was parallel and less near the heart.
Just a thought in at times and dependant upon age and lumber type, it might not have the grain coming together in a wave like thing such as this tool is designed to do.
Also as an observation. You might find that if you want your wood graining ever so much or just slightly more lively, you might add a layer of gold spray paint or gold powder to the mix in giving the wood a bit
of reflectivity in color and reality. Otherwise, perhaps cut down a bit
on layers of gloss and or perhaps a bit
of talc powder or baby powder to sort of dull the graining - again just ever so light that coating. Neither of these your painting, just more an effect
on it to liven it up or tone it down as in optimum conditions perhaps added to one of perhaps a few layers of gloss coat that goes between layers of paint. This if not wet brushing or with wet brushing, perhaps added while it's still tacky.
Wet brushing from my experimentations so you get the variation in color between color choices but some lack of ability to control graining. Perhaps wet brushing if closer to the audience... perhaps but two very different techniques.