I really can't answer that.If it's a spot of steel the size of a dime or more I'll put a light coat of rattlecan etch primer over it before I seal it (or a squirt of plastic primer on plastic) but if it's bodywork I broke through to I like to put 2 normal coats of the same primer down for a scuff coat.I have sealed right over the spots but usually they'll soak in later and come back to haunt you.Technically the substrate you sanded needs to be a certain thickness to avoid swelling and keep a solid barrier between the old and new.When you see bullseyes and shrinkage this is part of the problem.Ideally when you work a part you should do the final blocking and lay down 2-3 more coats of primer (depending) for a scuff coat.Just knock about 1/3 of that off sanding and seal it.Treat old paint like bodywork too.If all you have is a broken edge of steel look at your sealer specs,some can hande small amounts of steel and these don't shrink or swell like paint edges or bodywork do.
