I have a wooden countertop that I covered with a thin layer of epoxy and, as a beginner, I made a messy job and there are brush strokes on it. Some days ago I started sanding it with a sander with different RPM’s. I started with a 180 grit and different RPM’s. In both cases, I turned my working area into a messy White-Christmas landscape. The low RPM option was less messy. However, I’m not sure which option is the “healthiest” for the layer. I assume that a high speed makes the abrasion hot, therefore destroying the epoxy layer (I also found this explanation somewhere), but is it an issue to even consider? Can anyone help me understand which option is “ideal” for sanding epoxy? Low or high RPM and why?
Unfortunately this is one of those things which comes down to the ever-regrettable it depends.
There are numerous variables that are important to any specific situation of sanding epoxy and they go beyond the basic question of high/low speed being better in general.
Normally I would have asked what sander you're using (not just the type, but the exact sander model can be important) but actually it doesn't matter given all the other variables you also have to take into account, including abrasive type, the grit(s) being used, user technique (hand pressure, dwell time).
The other key thing of course is the epoxy itself. There are so many epoxies on the market with a range of properties including final hardness, and the temperature at which they begin to soften....
I assume that a high speed makes the abrasion hot, therefore destroying the epoxy layer (I also found this explanation somewhere)
Affect yes, destroy no. Epoxies do soften with heat, but in general you have to get them very hot indeed for them to be damaged.
Since you were producing fine white dust at all the speeds you tried it seems that in your case speed doesn't matter. A fine white dust is exactly what you look for when sanding any clear finish — if you get excessive loading of your abrasive, corning, the finish gets gummy/sticky, starts to lift from the surface, these are all signs that you're trying to sand before the finish has hardened enough.
An alternative for future consideration
Scrape instead of sand to get to a reasonably uniform or completely uniform flat surface, then merely sand to refine that. In addition to greatly reducing the load on your abrasives, scraping produces little or no airborne dust, and is nearly silent; here's a little more on why scraping is better than sanding.
You didn't ask about the this but since you mentioned it along the way I thought I'd tackle it.
In both cases, I turned my working area into a messy White-Christmas landscape.
Might I suggest that a priority should be to look at improving your dust collection? Or implementing something if you don't have any yet ^_^
Instead of or in addition to the in-built dust-collection ports that every (?) sander has, it could be worth making a downdraft table/box. These have certain advantages, including not needing adaptors for every power sander you own if you have a selection, and of course anything like this can be used when hand sanding too which I think is a huge boon.
1 Despite the watermark this is copyright to Fine Woodworking, not that site which features 100% stolen source material ◣_◢
2 How to Build a Down Draft Sanding Table from WV DIY GUY on YouTube
4 Rockler Downdraft Table Plan in PDF format