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Old 02-08-2008, 23:28   #1
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Epoxy - grinding surface speeds

I've come to the conclusion that when rotary sanding epoxy with a (24-36 grit) disk that the surface feed can not exceed much more then 1200 feet per minute or around 365 meters per minute. That comes to around 5000 rpm's for a 2" disk or 2300 rpm's for a 5" disk. The epoxy seems to melt and stick to the grit. And of course the epoxy has to be fully cured.

I've been working frantically on building up my cockpit before the weather changes back to cold again. In the process I discovered that a slower speed then what most grinders turn will preserve the sanding disk extensively.

The first picture below shows a disk that was loaded up within 15 minutes of use with a regular 4" grinder (10,000 rpm's). The second picture shows a couple disks that lasted over an hour running much slower then the average drivers used for that disk. And they are still usable. I used a variable step down transformer to regulate the drivers.

The third is a picture of the cockpit to date partially faired in.
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Old 04-08-2008, 13:53   #2
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Delmarrey, Now that you have the really heavy stuff ground off, here's a trick I learned by accident. As the epoxy is kicking off, dip your hand into a bucket of water that has a little detergent added to it. Splash the not-quite-hard epoxy with the water and then rub it with your hand and fingers to smooth it out. It's like working with clay. It is especially helpful when you are doing the inside fillets because it makes them glass smooth, requiring only a light touch of 120 grit paper to make them primer ready. I learned this when we were working on a fin keel, upside down, and it started to rain. We figured, "what the hell, we can't screw it up worse, now", and discovered how cool this is. BTW, if you can find some 16 grit pads, they also work well for the nasty work you were doing earlier, and yes!, speed is the answer.

The other lesson I learned was after priming, I sanded TOO much or TOO well, putting a mirror glaze on the surface of the primer. The LPU came off in sheets when I hosed it off the following day. Bummer, but, as they say, another damn learning experience.
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Old 04-08-2008, 14:29   #3
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Delmarrey,

Have you tried using flap disks for bulk material removal. They do not load up, and are zirconium grit on cloth backing... so they last forever too. Though they do make dustclouds! 80 grit cuts faster than 24 grit on a random orbit sander... with no pressure on it. I think its sort of like machining, you can run it where the part gets hot, the tool gets hot... or the chip gets hot, the dust flies off like talcum powder. (Grin)

One of the things I've been finding is that for making a nice smooth surface, buttering on a little fairing compound acts like a trace coat block sanding a car. You see all the wrinkles and ripples go from purple, to pink... and finally to yellow when you hit glass or unthickened epoxy.

Also, skipping from power sanding to as rough a grit 3M gold on a super stiff 2 3/4 wide stickit longboard makes things flat faster than any rotary tool I've seen... Straight, 90 degrees then 45 degrees one way, 45 the other and its just about done! Almost works like a huge bodyfile. (Which works also work ok if the goo hasn't set up super hard.)

Bastard files and #2's also do a half decent job for shaping... haven't used them enough on glass to see if it noticeably shortens their life expectancy though!
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Old 05-08-2008, 04:06   #4
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Delmarrey, you might like to try a ZEC disc for bulk grinding of glass, they usually come in 4.5", 5" & 7" & outlast other discs by 10-20 times & cut hard until the're worn out. All the best with your grinding from Jeff.
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Old 05-08-2008, 05:25   #5
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Quote:
Originally Posted by Roy M View Post
Delmarrey, Now that you have the really heavy stuff ground off, here's a trick I learned by accident. As the epoxy is kicking off, dip your hand into a bucket of water that has a little detergent added to it. Splash the not-quite-hard epoxy with the water and then rub it with your hand and fingers to smooth it out. It's like working with clay. It is especially helpful when you are doing the inside fillets because it makes them glass smooth, requiring only a light touch of 120 grit paper to make them primer ready. I learned this when we were working on a fin keel, upside down, and it started to rain. We figured, "what the hell, we can't screw it up worse, now", and discovered how cool this is. BTW, if you can find some 16 grit pads, they also work well for the nasty work you were doing earlier, and yes!, speed is the answer.

The other lesson I learned was after priming, I sanded TOO much or TOO well, putting a mirror glaze on the surface of the primer. The LPU came off in sheets when I hosed it off the following day. Bummer, but, as they say, another damn learning experience.
I've always thought that water killed the hardener. Most epoxy companies advise against working in high humidity. From memory, over 70% for my stuff (west system) is advised against.

I have used it in higher humidities than that, and it worked fine, but I would'nt do anything structural on those days.

So I'm surprised to learn that actually wetting it doesn't stop it from curing. Although there are epoxies that can be used underwater, so it really shouldn't be a surprise.

Do you guys use peel ply? Saves a ton of sanding, especially if you're going to be adding more glass to a cured area. Makes secondary bonds much better, and saves acetone washing.
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Old 05-08-2008, 06:11   #6
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What they said: Peel ply means no lumps; a quick scuff and it is ready to paint. But peel ply does not guarantee a fair surface.
Spreading easy-sanding fairing compound onto the gelled-but-not-cured glass work results in an easy to longboard/sand surface after everything cures. This assumes that the resin in the glass and the fairing compound are the same basic type (either all epoxy or all polyester. I mix up my own fairing compound so this is never a problem for me, it is the same resin).
Delmarrey, although heat will cause the resin to soften and load up the discs, they should last longer than you are experiencing. The resin may not be fully cured. A heat cure or longer cure may help the situation.
However, if I am going to longboard fairing compound or wet sand epoxy paint primer, I get to work as soon as possible: the longer you wait, the harder it is to sand, and hand sanding with a longboard or wet sanding do not seem to cause the sandpaper to load up even with relatively green resin or primer.
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Old 05-08-2008, 08:49   #7
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The wet-forming of the epoxy is a surface treatment, and it works. Have any of you ever used Z-Spar Splash Zone epoxy putty? It is the greatest! You actually used wet hands, regularly dipped into water, to mix and mold this stuff. I have used it to form fairings and repairs of lots of stuff. I use it to fair the lower rudder hardware to provide clear laminar flow. When I haul out, every couple years, I break it off with an old chisel and a heavy hammer, inspect the underlying hardware for electrolysis, then remold it prior to bottom painting and launching. The lower gudgeon and pintle were installed about fifteen years ago, out of 316 stainless. There has been ZERO ELECTROLYSIS in that period. Of course, I do spread gingerbread crumbs in a counter-clockwise circle, three times around the rudder the night before the launch. In the morning, the crumbs have all been accepted by the menehunes and properly blessed and protected. I'll post some pics if anyone wants to see them.
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Old 05-08-2008, 09:00   #8
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I bought a Milwaulkee 9 inch variable speed Sander/Polisher 0 to 6,000 rpm, model number 6078. At nine pounds of weight you get a workout. After the first day, the muscles adjust. It did quite well on stripping off 20 years of bottom paint, down to the epoxy coat on my steel ketch. The low speed setting around 2,000 rpm did very well with flap and disc sanding in the 36 grit work. The paint spec calls for 80 grit, and that is also going well with 9 inch discs also at 2,000 rpm. The tool is set up to polish, and convert to a grinder as well. It was a superb investment at $220, with 150 feet of 12 guage power line was another 80 bucks or so.

In retrospect, I would have splashed in and moved to another yard that allowed sand blasting. A high cubic foot per minute air compressor with a blasting rig, 100 pounds of bagged abbrasive, and breathing apparatus, is complicated, but well worth the investment. Or if doing a major bottom job, just contract it out to a professional blaster. However, I'm in farming country and most of the components to sand blast with are available through the old boy network. I've not tried some of the alternate types of abbrassives, such as ZEK and the 3M sponge looking things. Although, next time around I'm sand blasting.
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Old 05-08-2008, 09:08   #9
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In the morning, the crumbs have all been accepted by the menehunes and properly blessed and protected. I'll post some pics if anyone wants to see them.
That would be great. I've never seen a Menehune.
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Old 05-08-2008, 09:14   #10
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Darn! The pictures are all black. Do you suppose the pesky spirits have a no-publicity clause in their contract? The after-effects pictures seem all clear, though.
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Old 06-08-2008, 20:38   #11
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I too was under the impression that water would ruin a cure . I know that if it rains (Seattle ) after a coating I usually have to sand out the milkiness (is that a word ? ). I know with Wests System they recommend that one washes of the residue that coats the surface after a cure, which loads the grit.

Zach, yeah I do use the small flappers for in the corners especially after they get a nice radius on the front corner. But for the flats they dig in too much.

Sanding - fairing compound? I haven't heard of that stuff for epoxy. What businesses sell the stuff? I use a soapy wax type compound for metals.

Quote:
Originally Posted by Roy M
Of course, I do spread gingerbread crumbs in a counter-clockwise circle, three times around the rudder the night before the launch. In the morning, the crumbs have all been accepted by the menehunes and properly blessed and protected. I'll post some pics if anyone wants to see them.
MY tradition is to mix a little blood in with each batch . 1) The boat becomes a blood sister. 2) If it ever gets stolen I have proof it's mine. 3) Hopefully the boat will never betray me..............................._/)
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Old 06-08-2008, 22:01   #12
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Hey Delmarrey,

Fairing compound = Thickened Epoxy (Microballons and cabosil, or West Systems 407.)

Plastic body fillers also fit the bill. Non structural stuff used under the paint to bring an object to perfection, fast. Bondo is the lowest common denominator... there are a lot of great other products on the market.

Glazing compound = half way between tooth paste and heavy creme. Used for spot fairing the little pin holes and minor imperfections. 3M and evercoat make lovely glazes... Just a touch thicker than ketchup leaning hard on the microballons is close in epoxy.

Spray with a coat of highly contrasting (navy blue or black generally) cheap lacquer or primer... longboard and it shows the low spots. Go back and fill... Thats the Trace coat.

Then a coat of high build primer that gets wet sanded to perfection... finally painted, and your part looks like it was dipped in glass.



Thickened epoxy is quite nice to work with... but compared to whats available in the automotive industry... it takes 10x as long to dry, and is like sanding rock candy. In order to get the air bubbles out you have to fold the thick (peanut butter...) epoxy with a paddle against the side of the bowl... or on a horizontal surface. Even then it drags and leaves pin holes. The good automotive stuff doesn't do that! Evercoat Rage Gold... Is the Rolls Royce of plastic body fillers! I'll be calling them tomorrow to see if it plays nicely with West Systems and Awlgrip. (I'm a little frustrated with the cure time of the epoxy... Drags a job that ought to take a half our and turns it into a day!)

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