I would first make sure you use the tape for reinforcement for the spar but I have used Econokote on foam wings with great success. It only goes on with a low heat setting and goes on very smooth to foam directly. Color choice is limited, however. I built a lot of foam wings for combat planes and I always cover either with econokote or ripstop nylon. The ripstop is useful for those high speed mid-airs which you should not have a problem with so I would try the econokote. Good luck.

Bill
Manassas VA