Tricks for sanding things smooth
If you start at 120 grit, proceed to 150 (you might be able to skip to 180), and finish up with 220. If you're going to stain, depending on the type of wood, you could stop at 150/180. If you’re using an oil or wax, proceed with 320, 400, 600. You will have to evaluate all this as you go, depending on type of wood and finish.
If you are running a professional furniture or cabinet business, hire a professional wood finishing consultant to create a new system--one that takes into account market target, type of wood, VOC regulations, shop size, staff, quality, ect. You will save money and reputation with this investment.
Replace the pad on your orbital sander if it is not clean and in good condition. Most of your white wood sanding should stop at 180 (or 220 on mahogany). Use a garnet paper, rather then a stearated, for white wood sanding, and stay away from aluminum oxide.
For an oil and wax Danish type finish, I like Watco, but it dries slowly. Allow these types of stains to dry thoroughly before the next coat. A light sanding of 320 between coats will help. For a low-gloss, hand-rubbed look, look at some low-sheen lacquers.
From the original questioner:
We’ve been doing three coats of Watco with 600 in between. The drying time is killing us, and altogether the finish seems insubstantial. We’re now trying 100% tung oil and also some blends.
Overnight air-dry is expected for an oil finish. The use of a little VM&P naptha may help the dry time some, but will cut the oil. I believe an oil finish is not as good as a lacquer. I think you have to reduce tung oil with turpentine. Force-drying the oil in an oven should help your dry times.
Make sure you are getting enough CFM to your sander. Don't polish sand your wood--finish has trouble sticking if there is no "tooth".
The cause of swirl marks in orbital sanding (air sanders) is often insufficient airflow. Most air machines require that you maintain at least 90 psi under load for each machine you run. When the psi drops below that, a drag is created and a swirl mark results.
You can put an air gauge between your sander and air line to see what pressure you are at when running. Also look at your fittings, couplings, hose diameter, distance from air source, etc., as all these factors effect what your actual psi will be.
For example, having a great air source 50 fifty feet away from your sanders and using small couplings and hoses will lose the benefit of the "great air supply," due to distance the air has to travel and restricted air flow/pressure.
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