When you say "gap", Do you mean a valley along the seam after the edge paint dries? Are you making fancy items? For something with layers like a wallet, I glue two pieces together, I usually make them both oversized and then mark and trim them. The result is an even and square cut. Depending on the item, I will cut one piece to size and the mating piece bigger then trim it to size. There are better edge finishes than what you are using. Buy some Giardini Basecoat Dense. It is a primer and filler. Apply a coat, let dry and sand. Use Uniters edge paint on top of the basecoat. I prefer Uniters over Giardini edge paint. Sand between coats. If you have a fileteuse, that helps by heating it and smooshing it around.
https://www.buckleguy.com/giardini-basecoat-dense/
https://www.rmleathersupply.com/collections/dyes-stains-finishes/products/uniters-heatable-edge-paint-matte