Hey Jackal,
The lines you are leaving may be a result of how you cut the rubber if its a replacement, if there is a small amount of "loose" rubber trailing on the end it could leave a line.
I find the best way to get a nice clean cut is to use anvil style pruning shears, (just a cheap pair will do) but anything is fine as long as you are left with a clean square cut.
As for time to clean an average property, it takes as long as it takes till you are happy with the job. speed will come gradually, but without doubt you will get faster. Try not to worry about the time it takes and just concentrate on the job and before long you will be churning them out.