I get a lot of questions about finish problems, and, as you can imagine, they are often difficult to diagnose simply from the description. In these situations, a picture can really be worth a thousand words.
Take the example shown here. How would you describe it? The situation really happened, but the picture is mine. It’s a picture of a flaw called pinholes.
These are tiny bubbles in the finish that turn into pinprick-size holes when you sand the surface level. You can tell the problem is pinholes and not bubbling caused by dust nibs or too much air pressure when spraying if all the bubbles are located just over pores. Pinholes occur most often in larger pored woods such as oak and mahogany.
The cause of pinholes is air bubbles escaping out of the pores and into the finish after the finish has set up enough so it can’t refill the holes. Pinholes are usually limited to fast-drying finishes and are more likely to occur when coats are applied thickly rather than thinly, and when the wood is warmer than the finish or it’s hot in your shop.
The best way to avoid pinholes is to spray very thin dusting coats at first. These coats dry fast and bridge the pores. If you’re using lacquer as your finish, it’s best to use vinyl sealer or shellac for the dusting coats because these better resist being redissolved by subsequent lacquer coats.
You can also avoid pinholes by filling the pores with pore filler and letting it dry thoroughly. Then apply your first coat or two of finish relatively thin so they dry before having time to break through and release air from the pores.
Once pinholes have developed in a finish, the problem is difficult to fix short of removing the entire finish and starting over. But if the finish is still thin, you can try dusting a coat or two (again, best with vinyl sealer or shellac) over what you’ve already applied, then continue with fully wet coats.
– Bob Flexner