Porter-Cable's new 390K 5-inch random orbit sander looks different because it is different. It is built around a new type of DC motor that Porter-Cable calls the EnduraTech. Besides not having brushes to replace, the motor has 71% fewer wear components, which should result in much longer tool life, according to the company. This DC motor also gives the sander more torque and a much shorter profile, much like the look and low-slung balance of pneumatic, random orbit sanders commonly used in manufacturing industries.
Feel is important in a tool that spends a lot of time in your hand. The squat design makes the sander a little harder to hold than taller models because there is no pronounced undercut in the body to wrap your fingers around, and the grip is fatter than most. This mainly affects lifting the tool, though it felt pretty comfortable while sanding. The rubberized surface on the top and sides helps keep a grip, especially for dusty hands.
There is a speed control dial at the rear left of the tool, but as a lefty, I couldn't reach it during use. I had a hard time turning off the center-mounted power switch, too, but at least the location of these controls kept me from accidentally turning off or changing the speed of the sander while working.
The sander has an 8-foot cord, which is nice but, in my opinion, you can never get that extension-cord connection far enough when sanding long boards. The tool also comes with a nice dust collection canister and small case.
In finishing out a custom house, I found the perfect opportunity to spend a lot of quality time with this new sander while finishing a staircase and constructing a hardwood deck. I started with the laminated bamboo treads, which involved a lot of flat prep work on hard material.
Beginning with 60 grit hook-and-loop paper, my first order of business was to address surface imperfections. The tool's lower center of gravity made it a snap to roll up on its edge and feather out any nicks or deep scratches, and then back to flat to blend them in. Unlike an older generation sander that I had along for comparison, I could not stall the pad's rotation or bog down the motor of this new unit. It is noticeably stronger, and I had to be really careful with the 60 grit so I didn't hog away too much material.
Besides having a bigger 3.5 amp motor, this new sander has an electronic feedback motor control that adds more juice when the tool is under load so it maintains its set speed. In fact, the motor is so strong that I ran into a new problem: The sander would move the workpiece, vibrating it under hard pressure. After moving through successive grits, I noticed the dreaded swirl marks after the 220 grit finishing. I determined it was because the sander moved the stair tread. On subsequent pieces, I achieved the same good material removal with better control by starting with a much finer grit. Although the paper didn't last as long as the coarser type, I preferred the control it provided. For aggressive stock removal or work with light pieces with this unit, I recommend securely holding down the wood. When I sanded 400 square feet of tigerwood for the master bedroom deck, I had such no problems because the boards were already installed.
With the stair treads off to the finisher, it was on to the handrails and stringers built from reclaimed teak. They were already installed, so this was a good test of the sander's maneuverability and control. Sanding materials in-place can be tricky, and to accomplish this effectively, I found the variable speed feature invaluable. Speed can be modulated from 7,000 to 12,000 orbits per minute. At higher speeds, it is easy to cover a lot of area fast, but the centrifugal force of the rapidly spinning motor made it fatiguing to rotate around contoured surfaces.
The short head provides a lot less leverage than taller sanders against this gyroscopic force, but it does offer a real reach advantage on installed handrails. Since I needed to maintain the edge profiles and didn't want to sand it all by hand, reducing the speed made all the difference. I used 220 grit and slowed the sander all the way down to get great control.
Again, I rolled the sander on edge and used just the very outside of the pad for accuracy. The slower speed made it a snap to control material removal and dress up chatter marks left by the shaper, while keeping a nice looking radius. At low speed, I also found it easy to just break the 90-degree edges consistently with a quick, light pass.
A porous plastic canister at the back of the sander handles dust collection. It clicks into place with a quarter-turn locking pin–much better that the older, slide-on style, which constantly vibrates off. The filter was pretty effective at collecting dust when the sanding pad was fully down on flat surfaces, but it let a lot of dust fly when used on board edges and rounded profiles, like most sanders.
At slower speeds with the motor moving less air, the dust collection also is less effective. During some indoor work, I connected a vacuum hose to the sander's dust port and had good results. The hose slid on snugly and stayed attached as I moved all over the work. There are connections for 1- and 1-1/2-inch hoses.
Overall, the new Porter-Cable sander is a nice unit. Once you've experienced the stronger motor, it's a must-have, and the variable speed feature adds a lot of control and versatility. At 21/4-inches shorter than the older models, this unit is compact enough to fit in some tighter spaces. A little less leverage to rotate the tool is a small price to pay for this access, in my opinion. I also appreciate the electronic motor brake that stops the pad very quickly, so you don't have to wait to set it down on your workpiece. I do wish it had a more defined finger grip. This sander is also available with a PSA pad under a different model number.
Eric Leoni is a freelance writer and craftsman in Boulder, Colo.