Author Topic: Tips for dealing with warped plywood?  (Read 5962 times)

Offline cgrfish

  • Third Mate
  • *****
  • Posts: 560
Tips for dealing with warped plywood?
« on: January 07, 2015, 01:48:56 PM »
Does anyone have any tips for dealing with warped plywood.   I'm to the point the only major components I need to cut out are the entry door, which I was just going to use 1/2 MDO for, and the cabin table.  I have two sheets of MDO left, one of which has a nice bow to it, and the other appears like it has waves in it ???  In normal spots I would cut them and count on being able to cross brace,etc, to hold the panel true, but on the door and table that really isnt possible.  Anyone got any great tips short of buying more plywood?
Jumbo 25
Start 5/2013
Flip 8/30/13
Launch 6/16/15

Offline GlacierBoats

  • Captain
  • *********
  • Posts: 3017
    • Glacier Boats of Alaska
    • Email
Re: Tips for dealing with warped plywood?
« Reply #1 on: January 07, 2015, 04:57:03 PM »
If you can manage just the right amount of preload, you can bend the wood so that it slightly bows the other way and then glass the (now) hollow side.  I tried this once, screwing the wood down with a couple of slim pieces of wood (cut out the glass over the screws later).  It worked for the most part, but it was still bowed and not so evenly.  I decided it wasn't worth the effort.  Probably better to buy new flat MDO and seal it up with epoxy before it warps...

Brian

The Great Alaskan - Professional performance - Easy to build! - https://www.glacierboats.com  ><((((º> ¸.·´¯`·.¸¸><((((º> ¸.·´¯`·.¸¸><((((º> ¸.·´¯`·.¸¸><((((º>

Offline Tebubaga

  • Boson
  • ****
  • Posts: 351
Re: Tips for dealing with warped plywood?
« Reply #2 on: January 07, 2015, 07:33:39 PM »
Anyone got any great tips short of buying more plywood?

Firewood. Don't even try to use it. It'll cause you way more heartburn than it's worth. I've got a couple bent cabinet doors from a warped piece of 1/4" AC ply that remind me on a regular basis that I can be a cheap bastard at times...  >:(
Russ (Juneau)
Built a J24. Extreme details at:
http://tolmanskiff.blogspot.com/

Offline jim shula

  • Captain
  • *********
  • Posts: 1721
    • Salt Water Workshop
    • Email
Re: Tips for dealing with warped plywood?
« Reply #3 on: January 07, 2015, 08:24:43 PM »
Exposure to heat on one side and moisture on the other might work.  I've done this on wide pine boards in the past.  Lay the warped wood on wet grass in the sun with the concave side downward.  The wood fibers on the concave side are shorter (drier) than the convex side.  They'll absorb moisture from the grass while the sun dries out the convex side.  Not sure how this would work with plywood, or even what the climate is in your location, but it might be worth a try.  Also I rarely see a piece of plywood that is perfectly straight when sighting down the 8' edge.  Maybe at least on the table you could incorporate some sort of stiffback to make it flat.

Offline Dave Wright

  • First Mate
  • *******
  • Posts: 1117
Re: Tips for dealing with warped plywood?
« Reply #4 on: January 07, 2015, 08:44:42 PM »
I haven't had much luck straightening ply. The only time I tried was with two doors, each about 18" X 48" on a big cabinet that I built - maple ply 1/2 inch thick.  I used a hot steam iron (wife didn't mind), rags, and rope around the panels tensioned like a bow with a block between the panel and the rope. I got the panels straight (all the while worrying about delamming them), but then they gradually reverted to the warped condition when they were unrestrained. I figured that I should add hard wood framing around them to straighten them, but never got around to it. Goodwill got the cabinet 20 years later.

Offline Vgager

  • Deckhand
  • **
  • Posts: 28
Re: Tips for dealing with warped plywood?
« Reply #5 on: January 07, 2015, 09:24:30 PM »
I worked in a plywood mill in the early 80s, when a specialty panel came out of the hot-press "canoe" shaped it was sold as shop grade. I spent 3 years laying up marine panels and my buddy worked downstairs in the scarf shop. (Oh if I'd only known I'd build someday.)

Offline larspa

  • Third Mate
  • *****
  • Posts: 670
    • Email
Re: Tips for dealing with warped plywood?
« Reply #6 on: January 11, 2015, 01:27:24 PM »
Tebubaga is right.  Don't bother.  In spite of my best efforts, I wound up with some warped ply (yup, you guessed it: from Home Depot).  I tried using stiffeners, I tried weights, but in the end I wound up with a piece of crap job and just tore the thing apart and started over with some good stuff.  A waste of time and money.  But my hat is off to anyone who has successfully flattened out a sheet of ply and was able to use it.

TE
Jumbo 24' launched Sept 2022

Offline smilin jack

  • Deckhand
  • **
  • Posts: 10
Re: Tips for dealing with warped plywood?
« Reply #7 on: January 18, 2015, 08:14:18 PM »
I worked in a plywood mill in the early 80s, when a specialty panel came out of the hot-press "canoe" shaped it was sold as shop grade. I spent 3 years laying up marine panels and my buddy worked downstairs in the scarf shop. (Oh if I'd only known I'd build someday.)
Vgager, I agree with the "should have" idea of stock piling reasonably priced plywood.

I worked in a major plywood factory for nearly 20 years. I was a panel patcher of marine plywood and got certified on running the scarfer. The boss let me make my own custom sheet one weekend for my new firewood trailer from 13-ply shop grade 3/4" marine plywood. It was 60" x 120". Shop grade was $0.03 per pound so got some for every boat project. I still have a few 48" x 168" 3/8 5-ply.

I've never had any luck trying to flatten out warped plywood.

I did see on a DIY slide-in camper build the guy made a light weight table from two panels of 1/4" plywood vacuum bagged with 2" pink foam as the core. The table edges were outlined in 3/4" pine lumber. He used contact glue like the old kitchen counter tops used. He put a block of plywood in the center core to attach the pedestal screws solidly. I've never tried it but hope to someday.

Dave ::)

Dave