The Rule (Sudbury) Elastomeric sealant is the only sealant that will adhere to Roplene. Other sealants may appear to stick but they are usually temporary in nature and will come loose. Here's a link to an online store that carries the Rule:
C.G.Edwards, Sudbury Boatcare Products
You can also order it through Merritt Marine, one of this site's sponsors.
Save yourself a headache and use the Rule.
Good luck.