Hi everyone – newbie, loving wood. So got brave an made a table for patio- 4 lovely oak panels, biscuit jointed, sanded, sanded,sanded…finished with poly. Imagine my horror – to find the end of the side panel at one end has bent downwards! On investigation I misread the poly “not for use outside”!!! So I have cut the offending panel off and am planning to buy another piece and fit it.
should I biscuit joint a piece to cross the ends of the 4 boards to reduce this issue happening – or brace them with cross boards underneath the table, also what finish does everyone recommend for outside.
ps I did watch a few videos and laid the boards alternately so the natural rings of the wood alternate.
Thank you very much for helping me
Nick
Replies
“[Deleted]”
I doubt that the poly itseld had anything to do with the movement. It probably said not for outside use because poly, especially water-based poly, does not hold up well outside. Try taking the top off, flip it over, and let the sun hit the other side. The table may straighten out. And if it doesn't, c'est la vie.
Agree 100%
Film finishes like poly for in the weather use are bound to fail. Look into oil finishes like what gets used on boats or penetrating finishes like what gets used on decks.
I would agree oil finishes have advantages for outdoor furniture, but they must be reapplied regularly. If that's not your thing use a marine or spar varnish.
As to the primary question regarding the wood movement, it can be a significant challenge even when building for indoor use and it gets magnified greatly when pieces are built for outdoor use. First problem may have started with your choice of wood "oak" is not generally a great choice for outdoor furniture, but there are many species of oak around the world. Here in North America red and white oak dominate the market with white oak being slightly better for outdoor use. Regardless with care you can get many years of service out of your table. Pay careful attention to where wood will touch the ground, make sure all endgrain is well sealed, if possible raise it above ground contact with rubber feet.
I am digressing from the question of how to prevent the the wood from warping again, the answer probably is you can't, if the wood wants to move it can apply tremendous amounts of force to joints, but you can improve your odds. The idea of a breadboard end is one way, but not with biscuits. Biscuits have there place in woodworking but not where strength is required. The biscuits would more than likely just snap under the stress if the boards started to bow as you described.
That leaves battens as your best weapon against a repeat occurrence of you problem, just remember to allow for wood movement. Two to three battens attached underneath the top should do the trick. Since your top consists of 4 boards they must be fairly wide and will need 2 screws apiece. Use washerhead screws installed in the center of a loose fitting 1" long slot cut parallel to the long edge of the batten. Snug theses screws down but do not overtighten them, remember they must allow the top to move as climate conditions change and the wood expands and contracts.
What @esch said.
Please post a pic though to be sure you are getting the best advice.
Yes, you need to post a pic! Outdoor furniture is a different animal. Cupping can occur b/c of the wood, or, very commonly, because it is not attached to the frame that allows it to move with weather changes. It has nothing to do with the finish.
The frame or structure has to be sturdy enough to counteract those forces, while also allowing for it those forces, if that makes any sense. IOW the top is firmly attached in a way to keep it flat (apron, battens, breadboard) yes allows it to expand and contract.
A bread board will help, but it won't totally prevent cupping. You can't attach it with biscuits, draw bored tenons is the right way (no glue!!).
IME, no finish lasts very long outdoors. Even spar varnish needs to be reapplied periodically. I've used "outdoor" oils like Penofin and Cabot, and sooner or later all wood turns gray, unless you use a stain (as in deck stain) with color in it. Even then, periodic reapplication.
Spar varnish is the best varnish for outdoor use, much better than polyurethane. Not because it is maintenance free, but because it is much easier to maintain. The poly will craze, crack and peel, requiring complete removal before refinishing. The spar varnish will chalk a bit, and will need sanding and re-coating periodically. All exterior finishes will require periodic maintenance. The clearer the finish, the more frequent the need.
Battens (with allowance for movement as previously described) are much better at keeping a top flat than breadboard ends.
Harvey
This forum post is now archived. Commenting has been disabled