Most of the time you end up removing too much material and cause the valve to ride unevenly in the casing. A company called American Grinding Products, aka USP, has many different grades of lapping compounds which are useful for fitting valves and slides. I use some of their products and find them to be be of very high quality. It is important not to use too abrasive of a compound since you want to maintain tight control on the amount of material you are removing. You can use household pumice if you want but be careful to notice what affect it is having on the valve casing.
Pumice has a tendency to break down during use becoming finer as it is used.
It is also hard to clean away from the horn. After all lapping, make sure that all the lapping compound is removed before reassembly. The oil will make it alot easier to move the parts when lapping. It also tends to keep the lapping compound in one place unlike water which allows the compound to spread all over the horn. Mail us any questions you may have concerning this topic.