Sandpaper Contamination

Non-Stearated Sandpaper. Important words.

I recently spent a lot of time trying to fix problems that I had no idea I was giving myself by not thoroughly cleaning off sanding dust from hand-sanding with Norton 3X stearated sandpaper.

Sandpaper normally clogs up, moreso if you are sanding a finish. Ways around that include wet sanding, or using stearated sandpaper which self-lubricates and resists clogging. The result is faster results that are more uniform than you would get otherwise. Norton, bless them, claims that their sandpaper is compatible with all finishes but my own experience says otherwise.

Once contaminated and poorly cleaned, shellac will form tiny pellets and a rough surface, while polyurethane will pull away from the contaminated areas. If poured on, shellac will fisheye as will lacquer. After laying down shellac thick, I've been able to come back with my thumbnail and run it along the finish, peeling it off with medium pressure.

It's possible to seal these contaminations in by repeatedly coating it over. The coats adhere very slightly and build somewhat horizontally to finally cover over the contamination, but it's best to just avoid this entirely. With polyurethane or varnish, when rubbing out, you can give yourself a real headache by burning through... since the contaminated areas are difficult to close up they tend to be easier to burn into since they may have a thinner set of layers of finish on top. And then once burned open the difficulty of closing them up raises its ugly head again.

The solution if you think you have a sandpaper contamination problem after you've started finishing, is to sand down the affected area, then wash it first with water, then with a 50/50 or 100% methyl alcohol wash. I've stuck pieces under running water and rubbed them.

The alcohol will help dry out the water, and then leave the piece overnight to be sure it is thoroughly dry before attempting to continue your finishing project.