If you sprayed the DPLF 40 over properly treated bare steel and it is smooth , you can lightly sand the DP ( if it has set 72 hours or more ) then spray color.

If the fire wall still has minor flaws and scratches , you can spray 2-3 coats of K-36 , let dry and sand , then spray 1 coat of the DP over the sanded K-36, then spray color.

Happy Painting :-)
SprayTech