Why do I need to apply so many coats of epoxy barrier coat on my boat?

Each barrier coat product has a designated application thickness. For a proper barrier coat, most manufacturers recommend a 10 mil dry film thickness (DFT) of product, which is the amount of cured product the manufacturer has deemed to be required for a proper barrier coat. For applications where a full barrier coat is not necessary, there is no designated number of required coats.

To achieve 10 mils DFT, when using a ⅜” nap roller, many brands recommend 4-5 coats. Some products indicate specifically to apply only a few very heavy coats, while Interlux Interprotect and TotalBoat TotalProtect recommend 4-5 coats. When brushing or rolling, it's easier to make a smoother finished surface by applying more coats with a finer nap roller. Other products boast only needing 2-3 heavy coats, but in the end, it may be a compromise on finish or having to sand between coats.

Barrier coat epoxy primers also work well as general purpose epoxy-based primers for decks, topsides, and other applications. Typically, only 1-2 coats are needed for these applications to get proper coverage and have enough material so that, when it is sanded smooth, the chances of sanding through are reduced.