Builders' Forum Archives
Re: sand paper grit
Posted by Mark Camp on Mar 18, 2005
Not quite correct, I'm afraid. The reason for sanding or "keying" the surface is to create a mechanical bond, not to reach uncured epoxy. If the epoxy is still uncured, you don't need to key the surface at all: you can just recoat.
The misconception: you are assuming that the epoxy under the surface is less cured. This is incorrect, I suppose. It has not had a lower average temp than the surface.
So how could it be less cured?
It has had about the same temp as the surface. (If anything, then a little higher, due to surface conduction, air convection, and evaporative cooling, which means the lower layer might be slightly MORE cured, I would speculate).
I think you are thinking of finishes where solvent evaporation is involved in curing. But epoxy curing is more like concreate curing--like concrete, epoxy cures just fine underwater--than like paint or varnish. Mostly a chemical reaction between the two parts that have been mixed, a reaction that does not use air and that is not very dependent on solvents evaporating, even they do use some solvent in typical formulations.
Air is actually a negative in curing epoxy, not an aid. It contains water and CO2, which react with the "hardener" (why they call it a hardener I still don't know) to create the contaminant called amine blush, which competes with the "good" reaction that is creating strong material.
In Response to: Re: sand paper grit by Gert Walter on Mar 18, 2005