I also use 505 and really like it. I have always sprayed on the top back and not the batting( do not know where I learned this). I lay the top on the batting,( right side up) pull the top half way down , spray, pull back and smooth in position and then do the same to thre other side. i also buy my 505 for Sew for less but I got a case (12 cans) for about $153 with free shipping. Sound like a lot of money but that was about 1 1/2yr ago and I still have 3 cans left. This stuff seems to go a long way.I really like it because there is little odor. As for a spot on the top maybe you did not shake can good enough or were holding it too close to fabric or at the wrong angle.