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we have had table tops with water fall legs that the hardwood cracks & or splits. on the seams and/or the center of a board. we tite bond the hardwood to a mdf substrate. Anyone else had this issue? or a solution?
Theres a thing called wood movement you might want to look into...
That thing called "wood movement" is actually moisture movement. It's especially problematic at this time of year. As the relative humidity drops moisture moves out of the wood and causes it to shrink. This is why it's cracking. You should check the moisture content of the wood you're using. Ideally, it should be around 8%. The higher it is the more likely you are to see checking and cracking.
What is unsaid above is that solid wood moves, MDF does not move. "Moving" actually meaning "absorbs or gives off moisture according to the environment the thing is in."
This is a fundamental. Bruce Hadley's book on wood tech would come in handy about now.
How thick is the wood you are gluing to the mdf? I’ve done a few tops where I’ve epoxyed both faces with wood. I usually use 1” thick plywood as the core, and glue on thicker wood to the faces. After it drys, I plane it down so the wood is about 1/8” or so thick on both sides. I glue it on thicker as it’s eaier for me to handle with out it breaking, especially if you have a long piece. Now your top is 1 1/4” thick and the applied wood is not much thicker than a layer of the plywood. This works well for when you have a “L” shaped bank of cabinets and you want to miter a wood top. The miter stays closed. I assume it would also work well for waterfall miters. Hope this helps and good luck
I second "Bruce Hadley's book." And The Wood Doctor's also. Seems you likely have a problem with differential movement between the wood and the MDF.
I agree with @Larry's response. It does sound like you may have differential movement between MDF and wood. I would suggest looking for wood movement related articles online for more help. Goodluck to you!
Agreed with the above.
Also, if it's a hardwood table top itself that's moving/cracking, you want to be sure that you're being balanced with your finish coats.
The general rule is "same finish film layer on top and bottom". However I admit to not always doing that. Usually when I'm spraying a table, the underside will only get 2 fairly heavy coats and I only do a brief scuff, not a true "smoothing". On the top, I will usually do 3-4 coats (conversion varnish or 2K poly depending on the situation).
That will help prevent the underside from absorbing ambient humidity (or drying out) at a more rapid rate than the top.
This may not be the "main" issue or solution but it is a small step closer to consistent success.