Glueing polyurethane coated plywood

[Pat Turner]

I'm making a drill press table out of scrap MDF core plywood where both faces have been finished with poly. I sanded (or scratched) the finish up pretty good with 80 grit on a RO sander, but I don't know how deep the finish penetrates. I'm reluctant to use PVA glue in case the veneer is still water proofed. The only other alternatives I can think of are urathane glue (gorilla glue) or contact cement. I want to use something liquid so as to get a thin layer to keep the pannels flat.

I'm assuming contact cement to glue laminate on the top. I'm also assumign with MDF I don't need to use ballanced construction (another sheet of laminate to stop the tabel from bowing.

I hate working with MDF, but I've gotten quite a few large offcuts from these $100 sheets of plywood,and I want the stability given our huge humidity swings in the south.

Thanks for any thoughts.

Pat

[johnny means]

Melamine glue works well on prefin.

[ian maybury]

Polyurethane is supposed to stick well to polyurethane, but careful solvent/cellulose thinners washing, then sanding and then final washing to remove dust are advisable.

What I'm not so sure about is how it would bond to what I'm presuming is a polyurethane varnish - the coating might not be a pure polyurethane. Some test pieces would consequently be advisable. Stuff like say wax or other treatments could also cause problems - although the washing and sanding should sort it.

Don't forget that with a liquid polyurethane glue that gaps are bad news (it foams into the gap - low strength), and the stuff likes to be fairly firmly clamped for same reason. (it also helps wetting)

I think it would be advisable to put laminate on both sides - MDF still moves a bit so far as I know. Contacts are pretty versatile - sanding and solvent washing should leave it fine, but again I'd want to test.

The biggest issue i've found in putting down laminate with contact adhesive is that it's hard to get the stuff down smooth enough with a brush, scraper or whatever so that it doesn't telegraph a little through the laminate. Spraying is probably the solution, but aerosol adhesive tends to be expensive and depending on the brand doesn't always spray well. Luckily unless it's very bad most of what shows seems to fade over a day or so - i'm presuming it creeps and/or shrinks as it dries but a better spreading method would be nice.

The other issue you can get with a contact is that it tends to gum up router cutters a bit when trimming. It's not a big deal, but maybe there's an alternative adhesive that would avoid this.

ian

[Pat Turner]

Thanks guys. It's not prefinished plywood, it's sheets of AB birch with 3 coats of Minwax fast drying poly from the borg. Long story, but I've had 20 sheets eating up space (They are not mine.) The owner wanted them poly'd for storage then she decided to cut a few to size and I kept the off cuts. I used a few for shelves, but I want to laminate them for the DP table, and maybe latter a router table. I hate MDF with a passion, so If I were to buy a full sheet, it would end up setting in the shop for ever.

I think I can clamp it pretty well. I do a lot of metal working, and I have quite a few 18 and 24" vice grip C clamps that will reach in the middle of the table, plus regular clamps and cauls near the edges.

[Larry Edgerton]

I keep Kangaroo Glue around for that kind of thing as I like to prefinish parts that will be difficult later. Its made for melamine but works awesome on almost anything.

Larry

[johnny means]

Be careful on these forums. They'll have you over thinking a really simple situation. Almost any adhesive would do. all you need to do is keep one piece of material from sliding off another. I've had tops stand up for years with nothing but double face tape to hold them together.

BTW, prefin doesn't necessarily mean you bought it prefinished.

[Van Huskey]

I keep Kangaroo Glue around for that kind of thing as I like to prefinish parts that will be difficult later. Its made for melamine but works awesome on almost anything.

Larry

Larry, have a link to this stuff? Sounds interesting.

[Pat Turner]

Be careful on these forums. They'll have you over thinking a really simple situation. Almost any adhesive would do. all you need to do is keep one piece of material from sliding off another. I've had tops stand up for years with nothing but double face tape to hold them together.

BTW, prefin doesn't necessarily mean you bought it prefinished.

I hear you. But I could see PVA glue not sticking at all, and PL2 not leaving a flat top. I've never used epoxy or cynoacrylic on something even close to this size. Other then contact cement and hot melt, thats my glue porfolio.
My other concern is strength of the table- If the sheet aren't glued together 2 sheets are twice as stiff as one. If they are glued together, they are 6 times as stiff. I've drilled PT 6x6s before on the DP, so I know I may have some weight on there from time to time. Plus I'm cutting 1/2" into the sheet for T track (but over cast iron table.)

Thanks,
Pat

[Jeff Duncan]

Keep it simple...you mention your worried about it still being water proofed and not allowing the glue to penetrate? Drip some water on the surface....if it absorbs over a couple minutes....it's not water proof;) If it repels the water even after several minutes than you can always sand a little more. Fast drying poly won't sink very far into the pores of the wood so once you get to bare wood you should be fine.

good luck,
JeffD

[David G Elliott]

http://www.rooglue.com/rooclear.html

[Rich Engelhardt]

I'm assuming contact cement to glue laminate on the top. I'm also assumign with MDF I don't need to use ballanced construction (another sheet of laminate to stop the tabel from bowing.
W/MDF you want to make sure all six sides are sealed/finished in some way.

Contact cement will work w/the laminate - the real question is do you really want something as slippery as laminate?
Slippery is good for a saw table - not so good for a DP table.

My DP table is made of melamine & while it's nice and slick & cleans up well, it takes more clamping than I care to do to keep pieces in place while I drill them.
Next time around for the top, I'm just going to paint it or poly it & maybe mix in some of that anti slip stuff used for floor paint.

[Pat Turner]

W/MDF you want to make sure all six sides are sealed/finished in some way.

Contact cement will work w/the laminate - the real question is do you really want something as slippery as laminate?
Slippery is good for a saw table - not so good for a DP table.

My DP table is made of melamine & while it's nice and slick & cleans up well, it takes more clamping than I care to do to keep pieces in place while I drill them.
Next time around for the top, I'm just going to paint it or poly it & maybe mix in some of that anti slip stuff used for floor paint.

I've got leftovers from a sheet of Wilsonart High Def, or 3D or what ever they call it. It's less slick than good cast iron. I hadn't thought of it but my 20" jet DP had a horrable grinding job, so bad it's not easy to brush saw dust off. I've smothed it recently with 80 grit sandpaper on a surface plate, but that rough finish may have spoiled me.

I'll make sure to seal the edges. I'm going to use maple on the front and sides, but I hadn't though about the back, which is radiused for the column. I'll make sure to seal it.

I've used rubber mats on a DP as well. McMaster sells rubber in sheets for gaskets and such and you can make a nice mat.

Roo glue, BTW, is esentially unavailable in GA. Amazon has it at a reasonable price. Main downside I saw vs PVA was limited shelf life. I'm gonna get some.

[Larry Edgerton]

Larry, have a link to this stuff? Sounds interesting.

I buy it from Baer Supply, a Wurth company. The bottle is out to the shop so I am no help right now. I buy it by the gallon, and it is not very expensive. A little more than Titebond, but not much. It last on the shelf for years, its thick, it does not run yet spreads easily, and like I said will stick to almost anything.

Larry