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I have always sanded plywood and solid wood with a random orbit sander using 80 grit, 150 grit, and then 220 grit. Yesterday I was working with a sofa table top that had been sealed and then sprayed with three wet coats of Hydrocote Gloss Polyshield, a water based polyurethane. In the past I had always just removed the gloss with a nylon pad and applied a coat of wax. This time I wanted to really flatten the final finish (to begin developing better technique). Sanded with a very flat wood block and 220 grit. Before all of the gloss was removed I had sanded through the finish (frustrating, but a good learning experience). I realize now that I could have sanded more lightly and then applied another coat of finish. I would like to ask the most professional approach to attaining a truly flat finish – spraying a number of coats of finish, or producing a more flat surface before beginning the finishing process. If better preparation, with what grit would I use a sanding block to level the wood, and if before the final grit would I continue using a sanding block through to the end? Trying to become a better woodworker. Thanks.
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If the table top is solid wood - you may get it perfectly flat today via a big drum sander - but it will develop slight dips and valleys quickly as the wood moves. This is especially apparent in woods like pine and oak. The grain lines actually raise up over time as the piece expands and contracts. I guess you could finish it very quickly after the initial flattening. Another thing is once you attach the top to the base - the screws will probably pull it out of flat.
If you really want perfectly flat - I guess you have to go with a plywood/mdf covered with veneer.
We're talking thousands of an inch but a typical coat of finish is something like 3-7 mil thick so even a slight dip or bump can cause real problems when sanding the finish. The edges are very susceptible to being sanded through.
I have recently begun using a product called micro-mesh sanding pads for my ROS. They work very well for this type of application - but you must have a thick coat of finish on to start with. The nice thing is that you can kind of slop on the finish and the ROS pads VERY quickly even everything out.
*In my opinion 220 is too coarse for sanding finishes. Wet sandind with 600 is a lot safer. After that try scotchbrite pads too adjust the sheen. These tend to conform to surface irreqularities without cutting through the surface.
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