Depends on the color....and yours is purple? BC/CC will "build" color with multiple applications. I would shoot a sealer, such as reduced DP50LF, over the original enamel. Actually, you should probably use a sealer of the same color that was used under your exterior paint. Using a sealer will do two things: first, reduce the likelihood of bleedthrough of anything under the sealer and eliminate compatability problems. Two, provide a uniform substrate on top of which to lay BC color.