Danger,
It's pretty simple and others may use a similar one. Get a long 6" to 8" 1/4" diam grade 8 bolt, cut off the head and threads to get a straight shaft. Cut a hacksaw blade width slot straight down on end of the shaft about 3/4" deep. Fit the shaft in the 1/4" chuck of your mini grinder. Take 12" long strips (can be varied) of 1" wide emory cloth (fine 240 or 320 grit) , put one end in the slot and roll the rest around the shaft in the direction your grinder spins. Place the roll inside the port and let er fly. As the emory grit wears the cloth comes off always leaving an emory edge to polish with. Wear eye protection and a dust mask and do it outside as it is a bit messy. Fast and easy and does a great job. The length makes it easy to reach deep into ports. The depth of the slot should aways be about 1/4" less than the width of the emory cloth you are using. Hope you can understand my description- can post a pic if needed. Cam.