If the old Dicor is not too dried out just apply over the top of the old stuff.
If it is hard and cracked a wipe with an acetone wetted rag (not sopping!) will make the surface of the Dicor "sticky" and the new Dicor will stick much better.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW