Use a little abrasive with a lot of oil in the first load and more abrasive for the next loads. When the lap is not effective, pop it out of the muzzle, rip off the old lap, remove the rod, clean the rod and bore, fix the string seal and pour a new lap. If you have done enough with the first abrasive, go to a finer one then a finer one until the bore is as bright as you want it. This will cut steel fast so you should be careful to not over do it. Crocus and Rottenstone cut much more slowly and are used for polishing after you have cut out the rough and tight spots.
Rouge works well for polishing also.
The correct stuff can be obtained from Brownells in Montezuma, Iowa and others. Try industrial supply houses also. Just be careful to not wear out your barrel by removing too much of that precious steel bore. These laps cut at any speed. Push and pull in a nice straight line at an even speed for the best results and the least wear on the barrel from the rod.