What is the most effective way to eliminate or avoid streaks in finish, particularly in the interior of hollow forms or bowls?—Jim Forbes
Donna Banfield replies: This is a difficult question to answer. It would be easier to narrow down the problem if I had photos of the turning, or even better, the actual turning. I regularly do a “forensic analysis” with my woodturning students by an examination of several of their turnings. We examine them together, discuss the problems they encountered, I share with them some techniques to avoid them and immediately go to the lathe to teach the student how to do that.
But we don’t have a photo or actual piece, so here are my thoughts. Some woods can be prone to streaking when a finish is applied, and it’s not always the finish, but the wood, or rather the surface. In all cases, your finish will be only as good as your surface preparation. That means 1) Scary sharp edges on your turning tools. As a tool dulls, there is a tendency is to push harder. This can mark, or bruise the surface, particularly on end grain. 2) Good quality abrasives, and only use them once. Don’t try to save money by re-using sandpaper. 3) Begin sanding with a low enough grit to remove the tools marks. That means 80-grit. 4) Use compressed air to blow out sanding debris in between grits. 5) Use water (plant spray mister) to raise the grain in between sanding grits at least a couple of times. 6) Have good lighting on your work with several different light sources. 7) Use mineral spirits at around 220 grit. Apply with a rag and wipe on the surface. Take the piece to a window or outside in the sunlight (remove the chuck from your lathe). This will simulate the finish, and show you any sanding scratches that would not otherwise show up until your first coat of finish. If you see any scratches, go back down a couple of grits and work back up again. 8) Try some hand sanding with the grain. Using the lathe to assist us with sanding, or using a corded drill to power sand has advantages. It speeds up a task that most of us dislike. But, it creates problems, too—those circular sanding rings. They will be most visible on the two end grain sections of the bowl. Stopping the lathe and doing some sanding with the grain will help eliminate them. The scratches will still be there, but because they follow the grain, they will be less visible.
Here is another possibility. If you apply your finishes while the turning is still on the lathe, while spinning, too high of speed and pressure from your hand can create rings. Speed and pressure create heat and friction. Once they’re there, the best way to get rid of them is to sand them out.