Our last Advanced Class of the year, and the guys were awesome!! Many of those present had very little time polishing paint with a DA polisher, so diving into wet sanding and rotary polishing was a huge step up for them. But they went from trepidation to comfort pretty quickly! Others on hand had a fair bit of sanding and rotary polishing experience but just wanted to hone their skills a bit more.
After the classroom portion of the program, it was out to the garage and a hands on demo before cutting the guys loose on panels and hoods.
We started out with hand sanding, discussing the finer points of the process, whether on flat areas or contours, in the middle of the panel or on edges.
Here we're pointing out the nice, tight overlapping we want to see when sanding a given area.
Very close spacing, with large overlaps, is often something beginners struggle with, but it's important when going after a uniform finish. And isn't a uniform finish kind of the entire point of this process?
Here we change up the angle of attack on the paint as we step down to a finer grit of sandpaper. This cross cutting ensures that you remove the previous grit marks before moving on to your next step. It also allows you to see your earlier sanding marks as they are in a different direction, and it also preserves as much of the paint film as possible by primarily removing the peaks of the sanding scratches you created in the previous step. If, for example, you are going to be sanding with 1000, 2000 and 3000 grit sand papers, each step should be cut in a unique direction. If you're only going to be using a single grit of sandpaper, it's still a good idea to do a second pass with this cross cut method so as to knock off the peaks of the sanding scratches and make your buff out that much faster and easier.
We also demonstrated some contour sanding. The E7200 Hand Pads are quite flexible and should be utilized in such a way that they conform to the contours of a panel. They also cut easily with scissors or a razor blade, and you should cut them down to appropriate sizes for details such as small areas on bumper covers, A-pillars, and other tight areas.
Edge work requires a bit more of a surgical approach. There is a cardinal rule in wet sanding - Do not sand where you can not buff! Since the paint on a panel edge is some of the thinnest paint on a car, it is recommended to avoid sanding over edges since you'll have to buff it out later, and that can quickly lead to strike through. It's certainly not a process for beginners!! Use your thumb to guide you along the edge and keep you just off it.
Next, DA sanding. The motions used with a DA sander are pretty much what you aleady know from DA polishing, including keeping the abrasive disc flat against the paint. Here we demonstrated how easy it is to tell when you are not flat against the paint: notice the footprints of the two passes at the top of this section - the first one is fully the width of the abrasive disc but the one below it is obviously cut off as evidenced by the narrow trail. That's bad enough when polishing paint, but when sanding it leads to a very uneven finish. Not good!
Below we've highlighted the full width (flat against the paint) pass versus the narrow width (up on edge) pass. There was still overlap, but this is a great visual indicator that you aren't keeping the sanding disc in full contact with the paint.
Sanded with a 1500 grit sanding disc, the finish is nice and uniform.
A second pass, here on the near side, with a 3000 grit finishing disc starts to reveal gloss once again. The far side is just 1500 grit as in the above image. Remember, gloss comes from flatness and smoothness, and 3000 grit is definitely far flatter and smoother than 1500 grit sanding marks, hence the appearance of at least some gloss.
With a foam interface pad between the abrasive disc and the backing plate, you can even address contours such as shown below. Notice how the interface pad is starting to compress as we push into the contour. By the way, that isn't dust on the left side of the shot, it's water droplets from our spray bottle.
Another angle of the technique used to address a tight contour like we have on this hood. We used a back and forth, into the contour motion to address this area. Notice, however, that we did not roll over the peak of the contour. To address the rest of this area, we would approach from the top side of the contour without going over the edge.
Once you're done sanding you have to remove the sanding marks. Traditionally this has always been done with a rotary polisher, a wool pad and a compound.
The wool pad will generally cut faster than a foam cutting pad, but the foam will generate heat faster. The dry edge of a foam pad can also be very dangerous if you get down into a contour and that dry edge contacts the paint, or a piece of trim. Wool is great for getting into these contours safely and efficiently. That's not to say that you should never use a foam pad for contours - just make sure the edge of your foam pad is not dry if it's going to contact the paint!
Quickly and easily buffed out down into the contour with M105 on a wool cutting pad. We ran the tool at 1500 rpm here.
Demonstrating working on edges with the rotary: since the tool spins the pad in clockwise direction it is important to understand how that pad is going to interact with an edge. From the operator's perspective, we're tipping the tool a bit to the right here (3 o'clock position) so that the rotation of the tool causes the pad to drop off the edge of the panel.
Same concept with the tool switched on.
This angle shows how the pad would ride up and onto the panel if the tool were tipped toward the 9 o'clock position (operator's left). Not only does this put a lot of friction into that very thin paint on the panel edge, but it can also cause the tool to be kicked back at you.
Some paint systems prefer foam pads over wool as the wool can mar them. Sometimes that trade off is fine since you can usually clean up the wool marks pretty easily with foam. Just another variable to keep in mind when rotary polishing on any vehicle. It's also why people will often have a preference for one pad material over another.
Demo over, it was time to let the guys cut loose with hand sanding, DA sanding and rotary polishing.
There's a reason we limit these classes to just 12 people - hands on guidance!
Always watching for technique issues.
Checking out his technique with the DA sander in the contours.
Earlier we stated that sanding mark removal was "traditionally always been done with a rotary polisher, a wool pad and a compound." Well, things have changed a bit in the past few years with the introduction of high torque DA polishers as well as advances in pad technology (think microfiber discs) and compound technology. So much so, in fact, that we can now use a DA polisher like the MT300 to remove sanding marks in many cases. It's a good idea to refine your sanding marks as much as you can before using a DA to buff them out, but generally speaking if you finish down with 3000 grit finishing discs on a DA sander you should be able to buff them out pretty easily with a DA. A good DA. And microfiber discs. And something like M100, M105 or M101. Here the MT300/DMC5/M105 are making short work of 3000 grit hand sanding marks.
First time use of a rotary polisher!!!
Another first time rotary user, working beautifully into the contours of this hood.