O.k heres the deal...astro turf makes the worst seams so you have 2 options,1 take a papermate pen and from the top of the carpet try to run a strait line threw the carpet separating the naps and making a clear path for your cheezy top cutter your useing,the goal is to not shear the naps when cutting the edge,and with practice you can run a strait line threw it,following the lines on the backing...
if that fails,which im guessing it will,you can surpentene the seam...over lap both carpets and cut threw both pieces in a half figuer 8 motion/supentene,this will help hide the strait line look,but if your knife bends and you shear and shave naps,it will look like a snake on your carpet,so set the depth proper on you knife and press hard to cut threw,but not too hard to tweek the blade...
if this fails,your running low on carpet and might want to buy some ozite carpet,which wears better and is cheaper and looks nicer and seams better...,but if you still have enough,call a installer in to finish,remember to seal the seam or your seam is going to fall apart in time,most installers dont even do this...if you still need help p.m me and ill give you my # and walk you threw it..heres 2 pics so you can see the separation of the naps...keep changing new blades every 10 feet,depending on what you cutting the carpet on top of...
