I assume this is the stuff also known as OSPHO? If so, paint, let dry, and paint again over areas where red showing through, and keep painting until no more red showing through. Then give one extra coast to be sure. can be used as a primer, . . . and yes, does start to harden when exposed to air, so don't get much more than you can use in a week.

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002