After a 2 year break, I'm getting back to my Shellback Dinghy project. I'm concerned about sanding the hull which has scores of epoxy spots from the drywall screws used to hold the laps together during the epoxy layup. I've sanded epoxy on wood before and always manage to get the wood surface lower than the epoxy.
Any tips or techniques would be appreciated...
Lee & Co.
Trying to be reasonable
Use a heat gun and scraper. Cured epoxy softens when hot, gets rubbery, and scrapes right off. It won't hurt the joints, and makes what would otherwise be a really nasty job almost pleasant.
[ 08-17-2004, 05:06 PM: Message edited by: Keith Wilson ]
Member #919
Scraper or chisel is what I'd try. It might help to heat the offending knob before attacking it.
Senior Member
When sanding something of varying hardness, use a block. I have 4 or 5 of the 3M rubber blocks and mark 'em on the back with the kind of paper that's in 'em. You can load several pieces of paper at a time. I usually use and entire sheet- four 2-3/4" x 9" pieces.
If sand, absolutly, use a hard block. But the scraper seems to work better for me. Another trick, if you plan to coat the job with epoxy anyway, it to do so before you cut the blobs off. Then everything is hard and sanding into soft wood is not as much a problem.
I third the block theory. If you've got a good bit of area, invest in one (or more) of the 3M blocks. If it's a few spots just wrap your sandpaper around a scrap of wood.
And while we're on the subject...
Fingers+sandpaper+surface = no no no
Use a block when you need to knock down uneven spots or really level a surface.
When you're fine sanding, wrap the sand paper around a pad. 3M came out with square sponge sanding pads a few years back. They're absolutely worthless for sanding, but you can cut them in 1/3 strips and then wrap your sand paper around the pad.
Using a soft pad evens the pressure. If you think I'm kidding or if this doesn't make sense, take a scrap of paper, imagine it's your sand paper and press it against an uneven surface - like the palm of your other hand. Still think you're sanding evenly???
Now with a piece of towel or something similar, create a pad between your "sandpaper" and your palm. Now the sanding pressure is spread across the pad and not concentrated where your fingers create pressure points.
- M
[ 08-17-2004, 08:45 PM: Message edited by: Concordia..41 ]
Member
Guys:
Stop with the sanding. Use a cabinet scraper to level the epoxy down to the wood level. It's much, much faster than sandpaper. A cabinet scraper is a simply a thin piece of steel approximately the size of an index card. You just have to file the scraper edge now and then to bring out a burr once the edge starts to dull down. This is not rocket science. Just put the blade in a vise, give it a few licks with a fine file until you've created a burr edge and you're good to go.
Scraping is far superior to sanding. I don't know why more people don't use it as their first choice. And in this age of looking for sustainable solutions, the scraper will outlast many hundreds of sheets of sand paper.
Senior Member
I would not use heat on epoxy spots where I want to retain some of the epoxy -- so I would not use it in your situation where you are just trying to shave off the epoxy that sticks up above the wood. I'd be concerned about the heat having undesirable affects on the epoxy that remains.
A scraper is great in the right situation, but in my experience it is a bit hard to apply to bumps that really stick up. For those I would first try to knock them down with a chisel. This is best done when the epoxy is still green. If these are 2 year old bumps be prepared for a good bit more work.
The sanding block technique is certainly a good fallback position, but in my experience it still usually leaves the epoxy a shade higher than the wood.
If the outside of this boat is to be coated with epoxy then the technique of applying the first layer of that coating before cleaning up the bumps might be a good way to go. When it comes right down to it, if the outside is NOT going to be coated with epoxy, and if the holes you are filling do not go all the way into the interior of the boat, then I might have used a more traditional (and softer) material for filling the screw holes -- but it sounds like it's too late for that. There is certainly no harm in using epoxy, it's just more work. If the holes do go through to the inside of the boat then a hard filler like epoxy is a good idea because failure of a softer filler in such a situation could create a pretty good leak...
Member
No heat. Card Scraper. Sanding block if needed.
http://media5.hypernet.com/cgi-bin/U...=1&t=009193&p=
http://www.cianperez.com/Wood/ WoodDocs/Wood_How_To/Smalser_on_TuningCardScrapers.htm
[ 08-18-2004, 10:51 AM: Message edited by: Bob Smalser ]
Why no heat? Certainly too much of a good thing is bad but while a little heat makes the epoxy soften the epoxy will reharden when it cools. If it is ever heated again it will take even more heat to soften it. While the room temperature curing epoxies we use do not require post curing it won't hurt it, it'll only make it stronger. Thus using heat in moderation to clean squeeze out from along joints is not a bad thing either. Besides wood is a good insulator. You'd have to scorch the wood to heat the epoxy within the joint enough to soften it. Seems to me.
Member
No need.Why no heat?
Makes it harder, not easier for me to do a clean job turning the surface to goo, first. Cured epoxy shaves clean, crisp and dead flush with the surrounding wood using a sharp scraper.
"No need."
I'll accept that. [img]smile.gif[/img]
Senior Member
I have used old machinist's files and shoe rasps. They work ok for the flat and outside spots.
Thanks to all who responded with suggestions. I think I've got enough epoxy bumps to try all the suggested methods. One thing I've got going for me is that I did use a chisel to take most of the epoxy off the bumps while they were still green.
The method of leaving them alone until after suface coating the laps never occured to me. If I epoxy coat the laps, I'll do this. If not, I think I'll give the cabinet scraper a try. I've used sanding blocks in the past and always managed to have the epoxy areas show up. I guess I havn't developed the right touch with the blocks. Hopefully a cabinet scraper will work better in my Ham Hands...Thanks again to all.
Lee & Co.
Senior Member
The other reason I said no heat was my concern that too much heat could damage the remaining epoxy. This may be overly cautious on my part, but I don't think it's completely invalid...I do not recall at what temperature you move beyond post-curing and into the damaging range but it seems to me that it's not THAT high...Originally posted by NormMessinger:
"No need."
I'll accept that. [img]smile.gif[/img]
Trying to be reasonable
It won't, at least according to the folks at System Three, backed up by my own limited experience. You have to be really stupid with the heat gun to get the epoxy inside the joint warm at all. Wood is a very good insulator; one can burn one surface of a lapstrake hull made of 6mm plywood, and the other side will be only barely warm to the touch (don't ask me how I know this, but I was having an very intersting conversation at the timeI'd be concerned about the heat having undesirable affects on the epoxy that remains.). Cured epoxy at 150 degrees F is nice and rubbery, not gooey at all. It scrapes off with very little effort, and doesn't stick to anything else. On can certainly get by without it, but I put removing excess cured epoxy without heat in the same category as hand sanding; something to be avoided if at all possible. Yes, I'm lazy, but life's short and there's a lot of other boat work to do . . .
[ 08-19-2004, 09:44 AM: Message edited by: Keith Wilson ]
Senior Member
Keith -- the one caveat I would note is that we are not talking about a joint here, where the layer of epoxy gets VERY thin and close to the wood that is such a good insulator (once you get to the epoxy that matters), we are talking about filled screw holes where there is a large mass of epoxy... none-the-less I would not argue much with the accusation that I am probably being overly cautious...
Member
I have just finished this phase on my Penobscot 14. As I filled the screw holes with epoxy, I put little squares of wax paper over the holes to keep the goop from running out. When the goop cured, I took the wax paper scraps off, knocked down any protruding epoxy with a rasp, and then finished them with a cabinet scraper. Worked beautifully.
Don
Senior Member
Duct tape works too for that Don. One time I made some little blue tape masks for screw holes by using a grommet punch. Could punch through many layers of tape to make a bunch of holes then just tore them off as needed. Was super easy to get a nice fill on the hole surrounded by the tape. Just a bit of rise above the wood. Peeled them after the epoxy had set. A little tape remained sometimes but it scraped or sanded right off. No stained plywood.
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