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View Full Version : Reducing Checking on Plywood????



Jess Potter
09-29-2004, 12:48 PM
Every time I cut a piece of exterior plywood :( , Which I shouldn't be using anyuways but cheap for experimental purposes, It checks like crazy. If I coat both sides in epoxy first will this reduce the chance of checking? Are there any tricks I should know. I try to go with the grain of the ply as much as possible but still have problems. Thanks alot, Jess

dmede
09-29-2004, 12:59 PM
Are you refering to the splinters that rip out as you cut the ply? I dont think thats really "checking" in the traditional sense (at least as I understand it).

If you are talking about the splinters that rip up with the saw blade you can minimize that by taping over your cut lines and cutting thru the tape or prescoring along the cut lines with a sharp utility knife.

HTH
Dave

Edited to reaspond to post below: Keith, I was going with his description of checking occuring as a result of cutting which makes it sound more like a tear out issue. My experience has been that checking in ply occurs as it is bent or as it cycles dry and wet but not as a result of cutting? Hopefully Jess can clarify.

[ 09-29-2004, 02:40 PM: Message edited by: dmede ]

Keith Wilson
09-29-2004, 01:25 PM
No, I think he means the small cracks that open up in the grain of the surface plies of rotary-cut Douglas Fir plywood. I have heard that this is, if not caused, at least exacerbated by the fact that theyíre now using smaller and lower-quality logs, so the veneers have to be bent quite a lot to make them flat.

The only foolproof cure I know of is fiberglass and resin. Epoxy alone certainly wonít do it; it probably delays checking a week or two. And I almost hate to mention this, but the only case I know of exterior fir that didnít check was a boat I built 14 years ago of ACX fir and polyester resin. I sheathed the outside in glass and resin, and slathered the inside (the C face) with polyester resin diluted about 20% with acetone (Brain cells? What brain cells?) Anyway, it resisted checking admirably, despite indifferent care, for at least ten years. In the last year or two it has checked a little, but not much. Polyester, BTW, does stick just fine to new clean ACX fir, thank you; it even seems to soak in a bit. I canít say Iíd suggest you do likewise, but in this one case it worked.

Keith Wilson
09-29-2004, 01:56 PM
dmede, reading more carefully, you may very well be right.

So, are we talking about small cracks that open in the surface veneer after the plywood is painted, or splintering along the cut edge as it's being cut?

Bruce Hooke
09-29-2004, 08:34 PM
If we are talking about surface checking after the fact then I think Keith has it right on.

If we are talking about splintering along the cut line then I would recommend the following:

1. Make a zero clearance insert for whatever saw you are using the cut the plywood (if this makes sense with whatever saw you are using...what kind of saw are you using?).

2. Make sure your blade is nice and sharp and that it is a high-quality blade of a suitable type. I couple of years ago I believe Fine Woodworking compared how a bunch of different tablesaw blades performed when cutting plywood.

3. Make sure your saw is well tuned up. This is especially true of tablesaws.

4. If the above do not solve the problem then consider scoring the line (tricky because you then have to make sure you saw right on the score line) and/or use the tape solution recommened by Dave. Be careful when you remove the tape because the tape can also lift splinters.

Finally, YES, epoxy coating the plywood before you cut it should eliminate problems with splintering during the cutting process. This can also be a very time-efficient approach because it takes much less time to coat a full sheet of plywood than it does to coat a bunch of small parts (of course you will still need to coat the edges). This is a perfect application for flow-coating (see the Gougeon Brother's book), which allows you to apply the equivalent of three coats in one shot.

Bob Smalser
09-29-2004, 08:40 PM
If you don't like goo, you can apply 50-50 linseed-turps hot from the double boiler until the plywood won't take any more.

Wait a couple weeks or a couple months then prime and paint with an oil based paint.

That's the traditional method to minimize checking in plywood.

JimD
09-29-2004, 09:13 PM
My experience, whether splintering or checking an epoxy coat will only be partially succesful, though a light sheath of cloth will be 100% successful. What kind of saw are you using, Jess?

Jess Potter
09-30-2004, 01:11 PM
I suppose I should elaborate. I guess using the the term "checking" was wrong on my part. What Im talking about is When using a jigsaw and circular saw I always get a "splintering" effect at the edges of my cuts. Maybe my blades aren't sharp enough or Im using the wrong kind of blade. Im not really sure. Thanks, Jess

Bruce Hooke
09-30-2004, 01:23 PM
Jigsaws and circular saws are tough when it comes to "splintering". Mlke has some good suggestions on the circular saws. For jigsaws I would try the following:

1. Make sure you are using a sharp, fine toothed blade.

2. If you can, cut down on the forward-and-back motion the blade makes when it cuts (some jigsaws allow you to adjust this). Less forward-and-back motion will result in a slower cut but should help with your problem.

3. My Bosch jigsaw has a clear insert that fits tightly around the blade creating what is, in effect, a zero clearance blade insert. If you have one of these (or if you can get one for your jigsaw) it would certainly be worth trying it.

B_B
09-30-2004, 08:54 PM
Many good suggestions - a minor point would be to try to ensure that the edge which will tear out (on a jigsaw it would most definitely be the surface in contact with the blade guide - the 'upper' side) is the edge which will get hidden when you place two edges together - your joint/corner/lap/chine etc.

kc8pql
09-30-2004, 09:03 PM
As has been stated in a few other posts, the best cure, other than a panal saw with a scoring blade, is to score your cut line at least thru the first venier with a sharp utility knife and then saw just to the outside of the line. This will work every time. It can't chip if it's already cut.

[ 09-30-2004, 10:06 PM: Message edited by: kc8pql ]

paul oman
10-03-2004, 05:09 PM
Think I will take a bit of plywood and coat sections of it with different epoxies - then leave out over winter....

Hopefully will have some results by spring!

paul oman
progressive epoxy polymers