If you have full access to your gutter corners needing reinforcing (or covering holes etc), then these will probably be ok. We bought 2 sets of internal and external gutter patches to use on 2 corner joins, to try and bury it in the waterproofer and reinforce the joints well. With a little overhang from some roof sheeting (some removed out of the way) both sides of the guttering, it was nigh on impossible to get it to stay in position and coat the top after bedding it into the liquid rubber. As said, it might work for some siuations. The liquid rubber is going to be coated 4 times, maybe 5, and hopefully will have the elasticity and longevity to do the job anyway.