I have a solid south american countertop that has a large cup in it right now and I don’t know what I did wrong or if it is repairable without re-sanding.
The material has been sitting in the shop for over three months. I milled the the 8/4 mahogany three times over three days. I glued up 7 pieces between 5 to 7 inchs wide making sure the grain was alternating. It was glued up on a flat surface with three sets of straight edges clamped across the grain for 24 hours.
Once it was unclamped it remained flat and I sand through the grits on both the bottom and top. I placed the countertop in saw horse for even air flow. I put two coats of a water based stain and three coats of varnish on both sides over the next week. It was still flat.
I came in this morning to apply the last coat and found the countertop with a large cup. The concave side on the top side. Any tips would be great. Thanks.
Replies
It sounds like you did everything right with possibly one exception, that you didn't choose stable boards for making the top.
I suspect you will find that most of the problem is confined to one board and that it is flat sawn with sharply curved growth rings from having been sawn from close to the center of the log, boards like this will cup sharply when they gain or lose moisture. Alternately you may have a board with wild grain created when the board was cut from a curved tree trunk or from a crotched log, again these boards are unstable and will cup dramatically when they are exposed to even moderate moisture changes.
There is nothing that you can do to make boards like this to settle down, no amount of care in preparation, glue up, or finishing will prevent them from warping later on, they simply can not be safely used in a wide panel.
The most stable glued up panel is made from narrow (under 3 inches), straight grained quarter sawn stock, the further you get from this ideal the greater the chance you will have warpage.
This is just a guess, but I suspect that something has changed in your shop over the past few days that either made the air dryer or moister than it has been on average over the past few months and this led to the wood warping. It may be that the panel will flatten out again with another change in humidity but it will continue to cup and flatten out each time the weather changes.
John White
Shop manager for FWW Magazine, 1998-2007
cm,
is there some way you can flatten the counter top while installing it to the cabinet? what i mean is, can you pull it flat by slowly bolting it down to a series of cleats in the top edges of the cabinet? i have pulled this off a time or two. as long as i was able to keep it in place, it stayed flat as it was held in check by means of the installing screws or bolts. the more cleats, the better.
once i built a breakfast nook table that was 4' square with the front corners cut at 45 deg. angles. there was a 3" apron on all six sides. the customer was very happy until the thing warped a full 1/2" from side to side. the problem was remedied via 1-1/2" angle iron pre-drilled and screwed to the table top bottom, across the grain.
stressfull.
eef
I have a two inch thick maple counter in my house that goes from concave up to concave down as the humidity changes with the seasons. If the underside is not exposed to the air, or just differently exposed to the air, this will happen.
I considered kerfing the underside and adding more attachment points, but I decided to live with it.
Bowling alleys have a 1" angle iron screwed to the bottom every 4 or 6 feet to prevent this. If you have room, that might be a solution.
The countertop will have a 13 inch over hang on one endgrain side and on one edge grain side. I didnt want to have any brackets on the over hangs so I glued up the countertop with three steel 1/2 rods in it. The holes drilled out for the rods are oversized for seasonal movement.
I scraped off the finish on the bottom of the counter (the convex side), put a heater on it, and clamped it with straight edges for a day and a half. Its a little better but not enough. I will end up having to put straight edges on the underside.
If you attach something cross grain to the underside of the counter you will need to allow for movement of about a full inch due to humidity changes, otherwise the counter will crack at some point in its first year of use. This is based on an estimated width of about 36 inches.Hopefully the oversized holes for the rods are oversized in length for the same reason.John W.
Check out how these guys recommend it to be done. It might help.
http://www.johnboos.com/support/countertop.cfm
Good luck.
I wonder if a series of T-slots, cut across the underside of the width and a steel T-bar tapped into place (but otherwise free floating) might help to keep it flat. Naturally, you'd have to add an edge board to cover the slots, though.
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