It's probably seam sealer, but unless it's cracking or chipping out, I'd leave it. If you feel its gotta come out, you can sand it or wire wheel it out.
If the paints not coming off and you don't have too many paintjobs on it, you don't need to strip it. Just scuff/sand the paint on top and go.
I just painted my 93 a few months ago, it was still the factory paint, and the only places I took to bare metal was a few dings in preparation for filler.
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'65 Mustang vert, 289 4V, 4 speed
'93 LX Ported E7's, Ported Cobra intake, 73mm MAF, 65mm Throttle Body, 1 5/8" shorty's, Underdrive Pullies, 3.55's, 5 speed snatched by a PRO 5.0 shifter, 155,000 and still screamin'!
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