using your brush just quickly back fill the letter with a dabbing motion without allot of care for going over the edges of the letter. force the bristles into the corners of the serifs etc. then using a soft cloth wrapped around the edge of say a nylon squeegee. drag across the tops of the high ground of the letters. ive used the edge of cutup card board in the past. this will remove most of the excess paint. then using a soft rag around your finger and dampend with something like meths or thinner, just wipe away any residue on the high ground. because its marble it would mark or damage anything.
do a small test on a single letter before you start, but ide imagine you will do it no sweat. depending on what you back fill with, will determine whats best to clean. i prefer meths because thinner often activates and makes the colouring run.
also when cleaning, rubbing, make sure you rub in one direction. not sure if this matters with the likes of marble but on surfaces like brass as it has a faint grain you can see abrasion if you cross it. so one direction rubbing.
been years ince ive done anything like this, sorry for being a bit vague. 😀