Sanding face frames properly
When you buy pads for your sander, your tool maker might carry the same pads in a series of different firmnesses. Look for firm or medium pads.
For spots like this I much prefer my Dynabrade 1/4 sheet sander. It's got a nice firm flat pad and weighs only about a 1/4 what an ROS weighs, which means much better control. They're not cheap, but sure are nice tools.
In general, a firmer pad would be the way to go. With good technique, it can be done with a soft pad, but it's tricky. With a hard pad, just always keep the sander flat and moving. Oh, and make sure the paper is tight against the pad - frayed sandpaper edges round corners off very nicely.
It's definitely technique. It's surprising just how the slightest uneven pressure can cause excessive sanding in one area and virtually none in the other, over the diameter of the sander. A couple of helpful techniques:
1. Use a carpenter's pencil to scribble pencil lines across the corner joint (and all along the frame if needed). Start sanding and watch carefully as the pencil lines begin to disappear. They should sand away pretty much at the same time along the entire area being sanded. If they sand off to one side, you will be clued in that you are applying uneven pressure to the sander and must make an adjustment in "feel" as you sand. If the pencil lines are coming off on both edges but remain in the center, the pressure may be even but it is too hard, effectively causing the sanding pad (and disk) to sand deeper along the edges. This is where PSA pads (which are not cushioned like H&L pads and paper) have the edge in flat sanding.
2. Sand the backs of the frames reasonably flat to 100 - 120 grit. Attach the frames to the boxes and screw adjoining cabinets together, mating the frames tightly like they will be when installed. Now sand the face of the frames flush at the center stiles and all the rest of the frame area except the outside stiles and about 8-10" of the rails on the side where another cabinet will butt against it. Unscrew these cabinets and put the next one up against the unsanded side and keep moving through your set of cabinets, sanding right across the 8-10” of unsanded frame that was left from the previous cabinet. Even if you do an uneven job of sanding, the cabinets will line up flush in the front when they are installed. I use this technique when I do a set of paint grade cabinets that will be installed and then painted on site. The edges between cabinet stiles remain crisp and flat, one against another.
What type of ROS are you using? Is it heavy? We just replaced all our ROS's with a light weight Dynabrade and it is night and day over older pneumatics and electrics.
I use air-powered 6" Dynabrade and have no problems. The problem is because either your pads are too soft or worn out or you're not handling the tools correctly. A little trick that works with my helper might work for you till you get the hang of it. I start the sander while it's resting on the corner, then I sand moving slowly in wider and wider movements till the sanding is done in that corner. Then I sand in the sections between. Another way is to lay dunnage around the frame 1/32 -1/64 thinner than the frame. Then the sander can't be tilted enough to tear the corners severely. Watch that the papers are not curled up at the corners, as this will tear in as well.
If you are using a Porter-Cable sander, the pad it comes with is very soft. This will cause the rounded edge problem. I only use sanders with a firm pad. You can always round over the edge yourself.
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