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Polyurethane help!

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Forum topic by ChrisCarr posted 02-07-2011 09:59 PM 1273 views 0 times favorited 11 replies Add to Favorites Watch
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ChrisCarr

196 posts in 2366 days


02-07-2011 09:59 PM

I am new to using polyurethane and am trying to apply it to mdf. My mdf was perfectly flat now after 3 coats of poly it is not flat anymore (checked with level). I am using minwax fast drying poly. Applied it with a foam brush. I sand in between coats. Only the last coat (3) has got gloss to it, the previous ones just absorbed and drakened the mdf. It needs to be perfectly flat as the mdf is part of my new router table.

Any input would be appreciated.


11 replies so far

View ChrisForthofer's profile

ChrisForthofer

150 posts in 2535 days


#1 posted 02-07-2011 10:15 PM

MDF does that with most any liquid except a true surface coating like paint. Even then some swelling is present, allbeit much less. Are you applying it to both sides of the top?

-- -Director of slipshod craftsmanship and attention deficit woodworking

View okwoodshop's profile

okwoodshop

448 posts in 2642 days


#2 posted 02-07-2011 10:32 PM

Chris, how about laminating formica to the top instead of poly? It will last longer and your wood will slide better.

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ChrisCarr

196 posts in 2366 days


#3 posted 02-07-2011 10:43 PM

I don’t want to pay $50 for a sheet of formica. I don’t believe laminate should cost more than my mdf top which was under $15 for an mdf half-sheet.

View Cosmicsniper's profile

Cosmicsniper

2202 posts in 2626 days


#4 posted 02-07-2011 10:52 PM

I wouldn’t finish it with anything, Chris.

-- jay, www.allaboutastro.com

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ajosephg

1878 posts in 3029 days


#5 posted 02-07-2011 11:27 PM

Chris:

I’d try to salvage it by putting three coats on the opposite side to balance out the stress. In the future alternate the coats. Coat side A, then side B, then side A etc.

In any case, you should always coat both sides so that it will interchange moisture uniformly.

The same principle holds for any coating on any surface, including laminates, but sheet goods are more sensitive than solid wood.

If after you coat both sides, it is still warped, it shouldn’t be a problem if the frame that you’re going to mount it to is co-planer. Just cinch it down to the frame with appropriate hold downs, and it will assume the flatness of your frame. The converse also holds, if you mount a flat top to a crooked frame, the top will assume the crookedness of the frame.

-- Joe

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ChrisCarr

196 posts in 2366 days


#6 posted 02-08-2011 12:06 AM

I don’t think the mdf bent, I think the polyurethane settled higher on the surface on one side than the other. Sanding helped it but i had to sand in certain spots.

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ChrisCarr

196 posts in 2366 days


#7 posted 02-08-2011 12:07 AM

I just sanded and put some thinned poly on.

View Jeff in Huntersville's profile

Jeff in Huntersville

404 posts in 2662 days


#8 posted 02-08-2011 12:19 AM

I don’t think polyurethane will be very slippery, even gloss, if that’s what you want in a router table top. I’ve never tried it on MDF but I wonder if good old Johnson’s paste wax will give you the same levelling problems? Could you try it on a scrap piece?

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ChrisCarr

196 posts in 2366 days


#9 posted 02-08-2011 02:06 AM

I was planning on putting paste wax on after the poly.

View Jeff in Huntersville's profile

Jeff in Huntersville

404 posts in 2662 days


#10 posted 02-08-2011 02:33 AM

Another thought is to use a layer of hardboard, 3/16” thick or greater. It has a very smooth finish, is inexpensive and can be replaced easily when you inevitably scratch it.

View ChrisCarr's profile

ChrisCarr

196 posts in 2366 days


#11 posted 02-08-2011 04:20 AM

I have already drilled slots for the fence mount, a center hole, routed a groove fitted with an aluminum miter track, and everything else.

I have done 3 full strength coats of poly, and 1 thinned coat. I looks good. Its un-level slightly. I can only fit just barely a playing card under the level.

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