I use the carpenters pencil for some cleanup as well. Sprue smears on the underside of the plate is about all you should use them for; any more and youve got stuff way too hot.
I dont find the issue exclusive to brass moulds though. Its usually an incident of mould temp AND alloy temp with flow being too high at the same time. Either back off the alloy temp or let the blocks cool a bit. Take care to note both when cutting the sprue. A half cut/smear is about right. A clean smear and base means there's probably too much heat in both the alloy and mould temp..
If alloy flow is too quick, along with the other instances, the mould venting will not keep up and the alloy will flow in those vents, causing fins.
Also note the blocks should also be shut tight and set to cool tight; keep the handle under tension and take care that the pins dont lose full engagement. If its smearing on top of the blocks when cutting sprue, odds are you will have tinning on the faces as well, regardless of block material.
If lead is all over the block faces every trace must be removed or else the same issue will continue.
I dunk my moulds until any top block residue is liquious and then brush top and faces with a fine bronze bristle brush until clean and let sit until the mould is agreeable.
Casting with lead on the mould faces will just make the issue magnify as casting resumes.
With tight pinned, moulds, always be sure the mould is fully closed and be sure when the mould is set down to cool you arent jostling anything loose.
The LEE moulds any shallow pins especially need close attention.
