The question was how to fatten up the buds. Not when he should flush and harvest. The products “dry kool bloom” or “beastie budz” by fox farms are similar that the are about 0-50-30 they are intended to be used in late flower to shock the plants into swelling and trichomes development.
Disclaimer! Do not do what I do....
I use them at 1/4 tea spoon for 4 gallons of water starting week 3 of flower. Mind you I am hydroponic. I maintain and go to a half tsp on week 5. Week 6 hours I use a full table spoon for 4 gallons and run that until I flush. Everything I read says NOT to do it but I got my info from an old timer and tried it with good results. Have a look and my stardawg and dark devil diaries.
Again I can not recommend that’s what you do. I’m just sharing my information with you.