If you want to make the pins invisible, make sure you use pin material that is the same as the bolster material. Not all brasses are the same, and not all stainlesses are the same. If you use 360 brass for the bolster, use 360 brass for the pin. Also, do not pein lightly, otherwise it won't expand to fill the entire gap. Let your pins stick out about 1/8" on each side. Then beat the living heck out of the pins with the hammer, until they sort of flatten. I've done that to my bolsters, and I can't find the pins after sanding them down... even in bright light.