I have always used a vibrator sander to sand my hot coats. I bought a Harbor Freight sander/polisher and used it for the first time last night. I noticed that when sanding, 100 grit 1st hot coat I do 2 hot coats, on one area the glass started to bubble up. I stopped and felt the glass and it was hot. Obviously the sander was turning over too many rpm’s. The bubbles seem to settle back on to the foam and slowed the sander down an finished. My question is will this cause a problem down the road? Also, do you use your electric sander to wet sand? Thanks in advance for you help. Frank
Hi Frank -
By ‘vibrator sander’, you mean something that takes a half or quarter sheet of sandpaper, right? I’ll bumble along on that assumption, anyhow.
Now, a few things about using a sander/grinder/polisher;
You may be used to kinda concentrating on one area with your orbital sander ( which is what the manufacturers call the ‘vibrating’ rig I think you’re describing ) and you can’t do that with an 8" disc turning fairly fast. You have to do relatively big , fast, smooth sweeps, so you don’t eat right into the glass with it. I try to keep about half the pad in contact with the surface I’m sanding, make sweeps about as wide as I can, given a comfortable working position and all. The disc isn’t in contact with any part of the glass for more than a fraction of a second.
This also helps a lot with the heat buildup. The disc isn’t in contact with the glass for any real length of time, so you don’t have a chance to really heat it up much, and it only gets a quick pass across. So does turning down the RPMs like you did, but still you have to make the long sweeps. One trick - practice on some plywood. If you’re sanding through one layer of it in small spots and not doing much to the rest, you need to work on technique.
It also helps with heat buildup on the disc itself - you may have noticed some kinda translucent smeg in between the grit on the disc. Well, that can be a wax plus resin buildup you get when things are running too hot. Two bad things that does: the smeg holds heat really well and worse, it creates lots of friction which makes the heat get worse, 'cos it softens the resin, which sticks to the disc making more smeg which heats up and… you get the picture. Sandpaper just cuts, it builds up much less heat when it’s running right. So, if you see some of this buildup starting, let everything cool off and either replace the disc or try one of those sandpaper cleaners, the ones that look like a giant art gum eraser. I have also gotten the stuff off by sanding some end-grain wood, like the end of a dry 2x4 clamped or set into a vise.
Will the bubbles be a problem down the line? Likely, it’s heated up and come away from the foam, and what’s more as it bubbled up you may have inadvertently sanded away some of the glass too. If they are just pencil-eraser size or so, certainly no bigger than a dime, and they are still solid to the touch ( relative to the rest of the glass) then you can maybe get away with doing nothing, but if they are bigger it’d be a good idea to carefully cut them out and patch the spot with some cloth. If you have a tint or airbrush there, you might try injecting a very little thinned resin in under it and a patch of thin glass over it to preserve the color.
Wet sanding with an electric sander - not the best move. The chances of getting zapped or at least shorting out the tool and your workshop are pretty high.
I have tried wet sanding with pneumatic sanders, the small jitterbug air-powered jobs that use a quarter sheet of sandpaper and the 6" disc/random orbit air powered types, but for the time, hassle and setup it really wasn’t worth it. I think you’d be better off using your orbital sander with the finest dry-type sandpaper you can find, then giving it a few light passes by hand with finer and finer wet sandpaper until it’'s ready for polishing.
Though if you wanted to see something comical, you should have seen me trying to operate an air powered random orbit machine with one hand, feeding a stream of water to it from a bicycle water bottle with the other, splatter and water and white glop everywhere as the sander was just on the edge of getting away from me. Real glad nobody did a video of that…
Lest I forget ( and please don’t ask how I found this one out - lets just say it was back to Square 1 on that job) - you can get a nice heat buildup with a polishing wheel and compound too, which makes bubbles like you’ve had already, so you want to use the same long, light passes technique you’d use with a sanding disc on the machine.
hope that’s of use
doc…
Thanks Doc, I appreciate the answer, you hit the nail on the head. I also appreciate all the answers you provide here, I have learned a lot from you and all the other regular posters. Frank
As the saying goes “a poor man can’t afford cheap tools” the best tool I ever got was a variable speed Snap-On sander, 0-2800 RPM. It can be used like a power sanding block, run only as fast as needed, the heat build up is from too fine a grit and staying in one spot way too long, friction=heat