When the tire blew on this trailer it got the darco as well. It also cut through it and the flooring above it. Between new flooring material (underneath), spray expanding foam and new darco (along with coroplast) they fixed it. It fixed it but I am going to implement John's fix as soon as I get back to TX in the spring. I drove 3 days in almost non stop rain getting here and I have very little faith in the darco (or the repairs) for that kind of intense exposure to water - I want my floor to stay intact!!
I can also attest that the scrim type tape is difficult to deal with. Along with being very "persnickety" in how you handle it, I usually can't even get the paper backing to start to come off!!
I use 4" as well. You can also buy 4'x4' (if I recall) sections of darco floor repair material from Amazon for areas that it would work on.