epoxy substrate question

U

underdog

Broke through 2k blocking with 320 back to fresh steel in a few dozen places, nothing bigger than a silver dollar, also had to fix a few low spots that eluded me when blocking my filler.

Anyways my question, spraying epoxy over my bare metal and filler areas will the 320 be enough to give good adhesion?

Ive read the tech sheets, and my first reaction would be rough the areas with say 80-120, but the more I think about it why?

If you can spray redeced epoxy as a sealer (which I always do) over blocked 2k say 600 grit for a matalic base,and acheive adhesion then what am I missing?
 
I would sand the bare metal only with a piece of used (not worn out) 80 grit. I think 320 on 2k would be fine for epoxy.
The harder bare metal requires a courser grit to scratch it than what a softer filler or primer needs for adhesion.
 
The harder the substrate is the coarser the grit needed to achieve the same scratch profile given the same sanding pressure. The larger abrasive is spaced farther apart than a finer abrasive so it applies more pressure to fewer cutting edges allowing it to cut better. The amount of pressure needed to produce the same scratch profile in steel with 320 vs. what is needed to put a full scratch profile in a softer substrate like primer is probably a 100X. Hope this makes sense.
 
That makes sense, I never thought about it like that.
These kinds of answers is exactly why I joined this forum.
 
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