Remove Residue NMP From The Wood

paint-and-varnish removers

To remove residue NMP, wipe several times with an alcohol-dampened cloth.

The paint-and-varnish removers commonly available in stores are gradually shifting from those available in metal cans to those available in plastic containers. The ones in plastic aren’t as strong or fast acting as those in cans, which are methylene chloride and various lacquer-thinner solvents. Just the packaging, plastic vs. metal, tells you this.

The vast majority of strippers sold in plastic containers are based on the solvent n-methyl pyrrolidone (NMP) as the active ingredient. It will be listed on the back of the container. This solvent is relatively expensive, so manufacturers often mix in other solvents to reduce the cost. But it’s the NMP that does the work.

The reason NMP is still effective even though it has less solvent strength is that it evaporates extremely slowly. It can therefore remain wet on the paint or finish for days if necessary to dissolve or blister the coating.

I find this paint stripper very easy to use when I’m not in a hurry. I just brush a thick layer of the stripper onto the surface and walk away for an afternoon, overnight, or longer if necessary, until I can scrape or wipe off all layers of the paint or finish at once.

The problem with this stripper is that some of the solvent remains in the wood for many days, and the solvent can retard the drying of the finish you apply. So, if you want to begin finishing right away, or even within several days of the stripping, you need to remove the residue solvent from the wood.

You could do this with heat, of course, to speed the drying, but it is usually easier to simply wipe with denatured alcohol, acetone or lacquer thinner. Alcohol is less expensive and less toxic. Wipe several times with a damp cloth to dry off the surface. You can feel if you have succeeded; the wood will feel dry. You can also tell if you haven’t succeeded; your sandpaper will clog up.

— Bob Flexner

Editor’s note – You’ll find all of Bob’s books in our store: “Understanding Wood Finishing,” “Flexner on Finishing” and “Wood Finishing 101.”