fistly mask up around the scratch so you dont get paint everywhere, try to come as close to the scratch as possible, then add thinners to your paint mixture, on a 100mls of paint youd add a few cap fulls of thiners into the mixure, dont add to much as you want the mixture to be a little thinck so it doesnt dribble, anyways try to find a nice art brush to apply the paint, dont DAB IT ON as you will leave it looking rough, try to apply a nice thin coat inside the scrath and let it dry for a few hours, then ater its dry add another layer till the scratch blends out. ( it might be a long shot but i usualy mask up the area and touch up panels with an air brush, then only cost about 25 bucks from bunnings if you have a small compressor its the best was to get the job done) brush isnt a bad option its just not the best. good luck